TypeScript Array

An array object stores a series of values using a single variable name.

Arrays are very commonly used.

Suppose you have a set of data (e.g., website names), stored in separate variables as shown below:

var site1="Google"; var site2="Example"; var site3="Taobao";

If there are 10, 100 of them, this approach becomes very impractical. In such cases, we can use an array to solve it:

var sites:string[]; sites = ["Google","Example","Taobao"]

This looks much more concise.

The syntax for declaring an array in TypeScript is as follows:

var array_name[:datatype];        //声明 
array_name = [val1,val2,valn..]   //初始化

Or directly initialize it at declaration:

var array_name[:datatype] = [val1,val2…valn]

If the array is declared without a type, it is considered to be of type `any`. During initialization, the array's type is inferred based on the type of the first element.

Examples

Create an array of type number:

var numlist:number[] = [2,4,6,8]

The entire array structure is as follows:

The first index value is 0, and we can access array elements based on the index value:

TypeScript

var sites:string[]; sites = ["Google","Example","Taobao"] console.log(sites[0]); console.log(sites[1]);

Compile the above code to get the following JavaScript code:

JavaScript

var sites; sites = ["Google", "Example", "Taobao"]; console.log(sites[0]); console.log(sites[1]);

The output is:

Google
Example

In the following example, we initialize directly at declaration:

TypeScript

var nums:number[] = [1,2,3,4] console.log(nums[0]); console.log(nums[1]); console.log(nums[2]); console.log(nums[3]);

Compile the above code to get the following JavaScript code:

JavaScript

var nums = [1, 2, 3, 4]; console.log(nums[0]); console.log(nums[1]); console.log(nums[2]); console.log(nums[3]);

The output is:

1 
2 
3 
4

Array Object

We can also create arrays using the Array object.

The constructor of the Array object accepts the following two values:

  • A numeric value representing the size of the array.
  • An initialized array list, with elements separated by commas.

Example

Specify the initial size of the array:

TypeScript

var arr_names:number[] = new Array(4) for(var i = 0; i<arr_names.length; i++) { arr_names[i] = i * 2 console.log(arr_names[i]) }

Compile the above code to get the following JavaScript code:

JavaScript

var arr_names = new Array(4); for (var i = 0; i < arr_names.length; i++) { arr_names[i] = i * 2; console.log(arr_names[i]); }

The output is:

0
2
4
6

In the following example, we initialize the array elements directly:

TypeScript

var sites:string[] = new Array("Google","Example","Taobao","Facebook") for(var i = 0;i<sites.length;i++) { console.log(sites[i]) }

Compile the above code to get the following JavaScript code:

JavaScript

var sites = new Array("Google", "Example", "Taobao", "Facebook"); for (var i = 0; i < sites.length; i++) { console.log(sites[i]); }

The output is:

Google
Example
Taobao
Facebook

Array Destructuring

We can also assign array elements to variables, as shown below:

TypeScript

var arr:number[] = [12,13] var[x,y] = arr //Assign the two elements of the array to variables x and y console.log(x) console.log(y)

Compile the above code to get the following JavaScript code:

JavaScript

var arr = [12, 13]; var x = arr[0], y = arr[1]; //Assign the two elements of the array to variables x and y console.log(x); console.log(y);

The output is:

12
13

Array Iteration

We can use a for statement to loop through and output each element of the array:

TypeScript

var j:any; var nums:number[] = [1001,1002,1003,1004] for(j in nums) { console.log(nums[j]) }

Compile the above code to get the following JavaScript code:

JavaScript

var j; var nums = [1001, 1002, 1003, 1004]; for (j in nums) { console.log(nums[j]); }

The output is:

1001
1002
1003
1004

Multidimensional Arrays

The elements of an array can be another array, thus forming a multidimensional array (Multi-dimensional Array).

The simplest multidimensional array is a two-dimensional array, defined as follows:

var arr_name:datatype[][]=[ [val1,val2,val3],[v1,v2,v3] ]

Example

Define a two-dimensional array, where each dimension's array has three elements.

TypeScript

var multi:number[][] = [[1,2,3],[23,24,25]] console.log(multi[0][0]) console.log(multi[0][1]) console.log(multi[0][2]) console.log(multi[1][0]) console.log(multi[1][1]) console.log(multi[1][2])

Compile the above code to get the following JavaScript code:

JavaScript

var multi = [[1, 2, 3], [23, 24, 25]]; console.log(multi[0][0]); console.log(multi[0][1]); console.log(multi[0][2]); console.log(multi[1][0]); console.log(multi[1][1]); console.log(multi[1][2]);

The output is:

1
2
3
23
24
25

Using Arrays in Functions

Passing as Parameters to Functions

TypeScript

var sites:string[] = new Array("Google","Example","Taobao","Facebook") function disp(arr_sites:string[]) { for(var i = 0;i<arr_sites.length;i++) { console.log(arr_sites[i]) } } disp(sites);

Compile the above code to get the following JavaScript code:

JavaScript

var sites = new Array("Google", "Example", "Taobao", "Facebook"); function disp(arr_sites) { for (var i = 0; i < arr_sites.length; i++) { console.log(arr_sites[i]); } } disp(sites);

The output is:

Google
Example
Taobao
Facebook

As the Return Value of a Function

TypeScript

function disp():string[] { return new Array("Google", "Example", "Taobao", "Facebook"); } var sites:string[] = disp() for(var i in sites) { console.log(sites[i]) }

Compile the above code to get the following JavaScript code:

JavaScript

function disp() { return new Array("Google", "Example", "Taobao", "Facebook"); } var sites = disp(); for (var i in sites) { console.log(sites[i]); }

The output is:

Google
Example
Taobao
Facebook

Array Methods

The following table lists some commonly used array methods:

No. Method & Description Example
1. concat()

Joins two or more arrays and returns the result.

 
var alpha = ["a", "b", "c"]; 
var numeric = [1, 2, 3];

var alphaNumeric = alpha.concat(numeric); 
console.log("alphaNumeric : " + alphaNumeric );    // a,b,c,1,2,3
2. every()

Checks whether every element in the array meets the condition.

 
function isBigEnough(element, index, array) { 
        return (element >= 10); 
} 
        
var passed = [12, 5, 8, 130, 44].every(isBigEnough); 
console.log("Test Value : " + passed ); // false
3. filter()

Checks the elements and returns an array of all elements that meet the condition.

 
function isBigEnough(element, index, array) { 
   return (element >= 10); 
} 
          
var passed = [12, 5, 8, 130, 44].filter(isBigEnough); 
console.log("Test Value : " + passed ); // 12,130,44
4. forEach()

Executes a callback function once for each element of the array.

 
let num = [7, 8, 9];
num.forEach(function (value) {
    console.log(value);
});

Compile to JavaScript code:

var num = [7, 8, 9];
num.forEach(function (value) {
    console.log(value);  // 7   8   9
});
5. indexOf()

Searches for an element in the array and returns its position.

If not found, returns -1, indicating that the item does not exist.

 
var index = [12, 5, 8, 130, 44].indexOf(8); 
console.log("index is : " + index );  // 2
6. join()

Puts all elements of the array into a string.

 
var arr = new Array("Google","Example","Taobao"); 
          
var str = arr.join(); 
console.log("str : " + str );  // Google,Example,Taobao
          
var str = arr.join(", "); 
console.log("str : " + str );  // Google, Example, Taobao
          
var str = arr.join(" + "); 
console.log("str : " + str );  // Google + Example + Taobao
7. lastIndexOf()

Returns the last position of a specified string value, searching backward from a specified position within the string.

 
var index = [12, 5, 8, 130, 44].lastIndexOf(8); 
console.log("index is : " + index );  // 2
8. map()

Processes each element of the array through a specified function and returns the processed array.

 
var numbers = [1, 4, 9]; 
var roots = numbers.map(Math.sqrt); 
console.log("roots is : " + roots );  // 1,2,3
9. pop()

Removes the last element of the array and returns the removed element.

 
var numbers = [1, 4, 9]; 
          
var element = numbers.pop(); 
console.log("element is : " + element );  // 9
          
var element = numbers.pop(); 
console.log("element is : " + element );  // 4
10. push()

Adds one or more elements to the end of the array and returns the new length.

 
var numbers = new Array(1, 4, 9); 
var length = numbers.push(10); 
console.log("new numbers is : " + numbers );  // 1,4,9,10 
length = numbers.push(20); 
console.log("new numbers is : " + numbers );  // 1,4,9,10,20
11. reduce()

Computes array elements as a single value (from left to right).

 
var total = [0, 1, 2, 3].reduce(function(a, b){ return a + b; }); 
console.log("total is : " + total );  // 6
12. reduceRight()

Computes array elements as a single value (from right to left).

 
var total = [0, 1, 2, 3].reduceRight(function(a, b){ return a + b; }); 
console.log("total is : " + total );  // 6
13. reverse()

Reverses the order of the elements in the array.

 
var arr = [0, 1, 2, 3].reverse(); 
console.log("Reversed array is : " + arr );  // 3,2,1,0
14. shift()

Removes and returns the first element of the array.

 
var arr = [10, 1, 2, 3].shift(); 
console.log("Shifted value is : " + arr );  // 10
15. slice()

Selects a part of the array and returns a new array.

 
var arr = ["orange", "mango", "banana", "sugar", "tea"]; 
console.log("arr.slice( 1, 2) : " + arr.slice( 1, 2) );  // mango
console.log("arr.slice( 1, 3) : " + arr.slice( 1, 3) );  // mango,banana
16. some()

Checks whether any element in the array meets the specified condition.

 
function isBigEnough(element, index, array) { 
   return (element >= 10); 
          
} 
          
var retval = [2, 5, 8, 1, 4].some(isBigEnough);
console.log("Returned value is : " + retval );  // false
          
var retval = [12, 5, 8, 1, 4].some(isBigEnough); 
console.log("Returned value is : " + retval );  // true
17. sort()

Sorts the elements of the array.

 
var arr = new Array("orange", "mango", "banana", "sugar"); 
var sorted = arr.sort(); 
console.log("Returned string is : " + sorted );  // banana,mango,orange,sugar
18. splice()

Adds or removes elements from the array.

 
var arr = ["orange", "mango", "banana", "sugar", "tea"];  
var removed = arr.splice(2, 0, "water");  
console.log("After adding 1: " + arr );    // orange,mango,water,banana,sugar,tea 
console.log("removed is: " + removed); 
          
removed = arr.splice(3, 1);  
console.log("After removing 1: " + arr );  // orange,mango,water,sugar,tea 
console.log("removed is: " + removed);  // banana
19. toString()

Converts the array to a string and returns the result.

 
var arr = new Array("orange", "mango", "banana", "sugar");         
var str = arr.toString(); 
console.log("Returned string is : " + str );  // orange,mango,banana,sugar
20. unshift()

Adds one or more elements to the beginning of the array and returns the new length.

 
var arr = new Array("orange", "mango", "banana", "sugar"); 
var length = arr.unshift("water"); 
console.log("Returned array is : " + arr );  // water,orange,mango,banana,sugar 
console.log("Length of the array is : " + length ); // 5
Other Extensions