Syntax for adding an element to a javascript array. Useful techniques for working with arrays in JavaScript


  • Translation
  • I. Iterating over real arrays
  • forEach method and related methods
  • for loop
  • Proper Use for...in loop
  • for...of loop (implicit use of iterator)
  • Explicit use of iterator
  • Using methods to iterate over real arrays
  • Convert to a real array
  • A note on runtime objects
I. Enumeration of real arrays On this moment There are three ways to iterate over the elements of a real array:
  • method Array.prototype.forEach ;
  • classic for loop
  • a “correctly” constructed for...in loop.
    • In addition, soon, with the advent of the new ECMAScript 6 (ES 6) standard, two more methods are expected:
  • for...of loop (implicit use of iterator);
  • explicit use of iterator.
    • 1. The forEach Method and Related Methods If your project is designed to support the features of the ECMAScript 5 (ES5) standard, you can use one of its innovations - the forEach method.

    Usage example:
    var a = ["a", "b", "c"]; a.forEach(function(entry) ( console.log(entry); ));
    In general using forEach requires connecting the es5-shim emulation library for browsers that do not have native support this method. These include IE 8 and above early versions, which are still in use here and there.

    The advantage of forEach is that there is no need to declare local variables to store the index and value of the current array element, since they are automatically passed to the callback function as arguments.

    If you're worried about the possible cost of calling a callback on each element, don't worry and read this.

    ForEach is designed to iterate over all elements of an array, but in addition to it, ES5 offers several more useful methods for iterating through all or some elements plus performing some actions on them:

    • every - returns true if for each element of the array the callback returns a value that can be converted to true .
    • some - returns true if for at least one element of the array the callback returns a value that can be converted to true.
    • filter - creates new array, including those elements of the source array for which the callback returns true .
    • map - creates a new array consisting of the values ​​returned by the callback.
    • reduce - reduces an array to a single value, applying a callback to each element of the array in turn, starting with the first (can be useful for calculating the sum of array elements and other summary functions).
    • reduceRight - works similar to reduce, but iterates through elements in reverse order.
    2. For loop Good old for rules:

    Var a = ["a", "b", "c"]; var index; for (index = 0; index< a.length; ++index) { console.log(a); }
    If the length of the array is constant throughout the loop, and the loop itself belongs to a performance-critical section of code (which is unlikely), then you can use a “more optimal” version of for that stores the length of the array:

    Var a = ["a", "b", "c"]; var index, len; for (index = 0, len = a.length; index< len; ++index) { console.log(a); }
    In theory, this code should run a little faster than the previous one.

    If the order of the elements is not important, then you can go even further in terms of optimization and get rid of the variable for storing the length of the array, changing the order of the search to the reverse:

    Var a = ["a", "b", "c"]; var index; for (index = a.length - 1; index >= 0; --index) ( console.log(a); )
    However, in modern JavaScript engines such optimization games usually mean nothing.

    3. Proper Use of a for...in Loop If you are advised to use a for...in loop, remember that iterating over arrays is not what it is intended for. Contrary to a common misconception, the for...in loop does not iterate over array indices, but rather through enumerable properties of an object.

    However, in some cases, such as iterating over sparse arrays, for...in can be useful, as long as you take precautions, as shown in the example below:

    // a - sparse array var a = ; a = "a"; a = "b"; a = "c"; for (var key in a) ( if (a.hasOwnProperty(key) && /^0$|^\d*$/.test(key) && key b)

  • If two values ​​are equivalent (i.e. their order is not important), the comparison function returns 0 (if a == b)

    • For comparison, the function uses array elements as its arguments:

    Function foo(a,b) ( //define the check function if (a b) return 1; return 0; //if a == b ) var a = ; a.sort(foo); //only the function name is passed as an argument document.write(a.join(", ")); //write the same thing more briefly var a = ; a.sort(function(a,b) ( //use anonymous function return a - b; //function returns value 0 )); document.write(a); //1,2,5,10 Try »

    The first entry in the example is written this way to make it easier to understand how it works. Notice how convenient it is to use an anonymous function in the second fragment. It is called only once, so there is no need to give it a name.

    Note: If there are undefined elements in the array, they are moved to the end of the array.

    slice method

    The Array.slice() method is used to copy a specified section from an array and returns a new array containing the copied elements. Source array it does not change.

    Method syntax:

    ArrayName.slice(begin, end);

    Array_name should be replaced with the name of the array from which you want to extract a specific set of elements for the new array. The method takes two arguments that specify the beginning and end of the returned array. The method copies a section of the array, starting from begin to end, not including end. If only one argument is given, the returned array will contain all elements from the specified position to the end of the array. You can use negative indices - they are counted from the end of the array.

    Var arr = ; arr.slice(0,3); //Returns arr.slice(3); //Returns arr.slice(1,-1); //Returns arr.slice(-3,-2); //Returns

    splice method

    The Array.splice() method is universal method for working with arrays. It modifies the array in place rather than returning a new modified array like the slice() and concat() methods do. The splice method can remove elements from an array, insert new elements, replace elements - one at a time and simultaneously. It returns an array consisting of the removed elements, if no element was removed it will return an empty array.

    Method syntax:

    Array_name.splice(index, quantity, elem1, ..., elemN);

    The first argument specifies the index in the array at which to begin inserting or removing elements. The second argument specifies the number of elements that should be removed from the array starting from the index specified in the first argument; if the second argument is 0, then no elements will be removed. If the second argument is omitted, all array elements from the specified index to the end of the array are removed. When using a negative position number, elements will be counted from the end of the array.

    Var fruits = ["oranges", "apples", "pears", "grapes"]; var deleted = fruits.splice(2,2); //returns ["pears", "grapes"] document.write(deleted); var arr = ; arr.splice(4); //Returns ; the array became: arr.splice(1,2); //Returns ; the array became: arr.splice(1,1); //Returns ; array became: Try »

    The first two arguments to the splice() method specify the array elements to be removed. These two arguments can be followed by any number of additional arguments specifying the elements to be inserted into the array, starting at the position specified by the first argument.

    Var fruits = ["oranges", "apples"]; fruits.splice(2,0, "watermelons"); //returns document.write(fruits); //became ["oranges", "apples", "watermelons"] var arr = ; arr.splice(2,0,"a","b"); //Returns ; became arr.splice(2,2,); //Returns ["a","b"]; became ,3,4,5] Try »

    It is worth noting that, unlike concat(), the splice() method does not split into individual elements arrays passed as arguments. That is, if the method is passed an array to insert, it inserts the array itself, and not the elements of that array.

    toString method

    The toString() method converts the elements of an array into a string using a comma as the delimiter character.

    Var arr = ["Milk","Bread","Cookies"]; var food = arr.toString(); document.write(food); //Milk,Bread,Cookies Try »

    Note that the method returns the same string as join() when called without arguments.

    indexOf and lastIndexOf

    The indexOf method returns the index of an element whose value is equal to the value passed as an argument to the method.

    Syntax of the indexOf() and lastIndexOf() methods:

    Array_name.indexOf(search_element, index) array_name.lastIndexOf(search_element, index)

    The first argument of the method specifies the value of the element whose index needs to be found, the second argument (optional) specifies the index from which the search will begin. If there are several identical occurrences, the smallest (first) index is selected. If an element with the desired value is not found, the method will return -1. Inside the method, strict comparison (===) is used for searching.

    Var a = ; a.indexOf(3); //return 2 a.indexOf(3,4); //return 6 a.indexOf(35); //return -1: there is no element with this value a.indexOf(2); // 1

    The lastIndexOf() method also returns the index of the element, the value of which is equal to the value passed to the method as an argument. The only difference is that the lastIndexOf() method selects the largest (last) index.

    Var a = ; a.lastIndexOf(3); //return 7 a.lastIndexOf(35); //return -1: there is no element with this value a.lastIndexOf(2); // 6

    Iterator methods

    The methods described below are iterators. In all modern browsers for working with arrays there are methods that are designed to iterate over elements and perform various actions above them. These methods are forEach(), map(), filter(), every(), some, reduce() and reduceRight().

    They iterate over the elements of the array starting from 0 to length - 1 and, if the element exists, pass it to the callback handler function.

    forEach

    Method syntax:

    ArrayName.forEach(callback, thisArg)

    The first argument specifies the callback function that the forEach() method will call for each element of the array. You need to write the implementation of the called handler function yourself. The called function must have three parameters: the first parameter takes as an argument the value of the array element, the second - the index of the element, and the third - the array itself. However, if you only need to use the values ​​of the array elements, you can write a function with only one parameter. The second argument - thisArg (optional) will be passed as the value of this.

    Var arr = ; function foo(value) ( ​​var sum = value * this; return document.write(sum + "
    "); ) arr.forEach(foo, 5); //the second argument will be passed as the value of this //example with three parameters var a = ; a.forEach(function(el, idx, a) ( document.write( "a["+idx+"] = "+el+" in ["+a+"]
    "); )); Try »

    filter

    Method syntax:

    Array_name.filter(callback, thisObject)

    The filter() method creates and returns a new array that will contain only those array elements for which the callback function returns true.

    Function isBig(element, index, array) ( //returns numbers that are greater than or equal to 10 return (element >= 10); //if the value of the element is greater than or equal to 10, the expression will return true ) var filtered = .filter(isBig) ; //filtered

    map

    The map() method creates and returns a new array, which will consist of the results of calling the callback(item, idx, ar) function for each element of the array.

    Var a = ; var b = a.map(function(item, idx, arr) ( return item * item; )); // b =

    every and some

    The every() method returns true if, for all elements of the array, the specified function used to check them returns true.

    The some() method returns true if one or more elements in the specified function return true during testing.

    Var a = ; a.every(function(x) ( return x 10; )) //true: one number > 10

    reduce and reduceRight

    Method syntax:

    array_name.reduce(callback, initialValue) array_name.reduceRight(callback, initialValue)

    The reduce() method applies the specified callback function to two values ​​in the array at once, iterating through the elements from left to right, while storing the intermediate result.

    callback function arguments: (previousValue, currentItem, index, array)

    • previousValue - the returned result of the callback function (also known as the intermediate result)
    • currentItem - current element of the array (elements are sorted in order from left to right)
    • index - index of the current element
    • array - processed array

    initialValue is the object used as the first argument of the first call to the callback function. Simply put, the value of previousValue when first called is equal to initialValue. If there is no initialValue, then it is equal to the first element of the array, and the search starts from the second:

    Var a = ; function foo(prevNum,curNum) ( sum = prevNum + curNum; alert(sum); return sum; ) var result = a.reduce(foo, 0); document.write(result); Try »

    Let's look at how this example works. The first arguments to the function foo are:

    • prevNum = 0 (since initialValue is 0)
    • curNum = 1 (current element is the 1st element of the array)

    1 is added to the number 0. This result (sum: 1) will be passed as prevNum the next time the function is run. And so on until it reaches the last element. The returned result is the amount from last launch, will be 15 (1+2+3+4+5).

    The reduceRight method works similarly to the reduce method, but it goes through the array from right to left:

    Var a = ["h","o","m","e"]; function bar(prevStr, curItem) ( return prevStr + curItem; ) document.write(a.reduceRight(bar)); //emoh
    

    2024 gtavrl.ru.