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TypeScript Closures Tutorial with Examples

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1- What is Closure?

In the TypeScript programming language, a Closure is a special function.
  • Similar to a function, a Closure is a block of statements with parameters and can return a value or nothing.
  • Unlike a function, a closure has no name. However, you can identify it through a variable.
Syntax:

// Closure syntax
function(arg1: data_type1, arg2: data_type2, argN: data_typeN) {
   // Statements
}
// Declare a variable as an alias for Closure
var alias = function(arg1: data_type1, arg2: data_type2, argN: data_typeN) {
   // Statements
};
The example below shows the difference between a function and a closure. Conclusion: a closure is a function without a name.

// This is a function called sayHello
function sayHello(name: string) {
    console.log(`Hello ${name}`);
}
// This is a closure (A function without name)
function (name: string) {
    console.log(`Hello ${name}`);
}
Basically, if you create a Closure that you don't use immediately, you must identify it, or else the compiler will report an error. For example:
Closure ID Example

// Compile Error!!!!!
function(a: number, b: number) {
    return a + b;
}
// OK
var closureId = function(a: number, b: number) {
    return a + b;
}
You may not need an identifier for a Closure if you create it and call it directly.
closure_ex1.ts

// Create a Closure and call it directly!
(function (name: string) {
    console.log(`Hello ${name}`);
})('Tom');

// --- Equivalent to: ---

// Identifier for a closure:
var sayHello = function (name: string) {
    console.log(`Hello ${name}`);
};
// Call closure via identifier:
sayHello('Tom');
Another example:
closure_ex2.ts

// Create a Closure and call it directly!
var result1 = (function (a: number, b: number) {
    return a + b;
})(100, 200);

// --- Equivalent to: ---

// Identifier for a closure:
var sum = function (a: number, b: number) {
    return a + b;
};
// Call closure via identifier:
var result2 = sum(100, 200);

2- Arrow syntax

TypeScript allows you to declare a Closure using the arrow syntax, which is also commonly known as Lambda syntax. Compared to the traditional syntax, the arrow syntax omits the function keyword and adds an arrow ( => ) between the parameter block and the content block.

// Arrow Syntax (Lambda Syntax):
(arg1: data_type1, arg2: data_type2, argN: data_typeN) => {
    // Statements
}

// Traditional Syntax:
function(arg1: data_type1, arg2: data_type2, argN: data_typeN)  {
    // Statements
}
Example:
closure_arrow_ex1.ts

// Create a Closure with Arrow Syntax.
var introduction = (name: string, country?: string) => {
    if (country) {
        console.log(`Hello, My name is ${name} from ${country}`);
    } else {
        console.log(`Hello, My name is ${name}`);
    }
};
// Call the Closure
introduction('Tom', 'USA');

// --- Equivalent to: ----

// Create a Closure with Arrow Syntax and call it directly:
((name: string, country?: string) => {
    if (country) {
        console.log(`Hello, My name is ${name} from ${country}`);
    } else {
        console.log(`Hello, My name is ${name}`);
    }
})('Tom', 'USA');
If the Closure's content consists of only one expression, you can write it more succinctly:

var closureId1 =  (arg1: data_type1, arg2: data_type2, argN: data_typeN)  => {
    console.log('Something');
}
// --- Equivalent to: ----
var closureId1 =  (data_type1 arg1, data_type2 arg2, data_typeN argN) => console.log('Something');

var closureId2 =  function(arg1: data_type1, arg2: data_type2, argN: data_typeN) => {
    return a_value;
}
// --- Equivalent to: ----
var closureId2 =  function(arg1: data_type1, arg2: data_type2, argN: data_typeN) => a_value;
Example:
closure_arrow_ex2.ts

// Create a Closure
var minus = (a: number, b: number) => a - b;

var result = minus(100, 200); // Call a closure.
console.log(`result = ${result}`);

// Create a Closure
var sayBye = (name: string) => console.log(`Bye ${name}`);
sayBye('Tom'); // Call a closure.
Output:

result = -100
Bye Tom

3- Define a function type

In TypeScript, the type keyword is used to define a new data type. In this section we will discuss how to use the type keyword to define a function type (Also called a Closure type).
Syntax:

type Function_Type_Name = (arg1: data_type1, arg2: data_type2, argN: data_typeN) => return_type;
Example: Define a function type that accepts a parameter of number type and returns a value of number type:
function_type_ex1.ts

// A Function Type:
type TaxCalculator = (amount: number) => number;
 
function function_type_ex1_test()  {
    var usTaxCalculator: TaxCalculator = (amount: number) =>  {
        return amount * 10 / 100;
    };
    var vnTaxCalculator: TaxCalculator = (amount: number) =>  amount * 5 / 100;

    var caTaxCalculator: TaxCalculator = function(amount: number)  {
        return amount * 8 / 100;
    }
    const AMOUNT = 1000;
    var usTaxAmount = usTaxCalculator(AMOUNT);
    var vnTaxAmount = vnTaxCalculator(AMOUNT);
    var caTaxAmount = caTaxCalculator(AMOUNT);

    console.log(`Tax amount according to US calculation: ${usTaxAmount}`);
    console.log(`Tax amount according to Vietnam calculation: ${vnTaxAmount}`);
    console.log(`Tax amount according to Canada calculation: ${caTaxAmount}`);
}
function_type_ex1_test(); // Call the function.
Output:

Tax amount according to US calculation: 100
Tax amount according to Vietnam calculation: 50
Tax amount according to Canada calculation: 80

4- Function type in parameter

Function Type can appear as a parameter of another function, closure, method, or constructor.
function_type_in_args_ex1.ts

// Define a Function Type:
type MyTaxCalculator = (value: number) => number;

function calculateTaxAmount1(amount: number, calculator: MyTaxCalculator) {
    var taxAmount = calculator(amount);
    console.log(`Tax Amount: ${taxAmount}`);
}
// --- Equivalent to: ---

function calculateTaxAmount2(amount: number, calculator: (value: number) => number) {
    var taxAmount = calculator(amount);
    console.log(`Tax Amount: ${taxAmount}`);
}
Example:
function_type_in_args_ex2.ts

// A Function accepts 2 parameters:
// 1 - amount: number
// 2 - calculator : (number) => (number)
function printTaxAmount(amount: number, calculator: (value: number) => number) {
    var taxAmount = calculator(amount);
    console.log(`Tax Amount: ${taxAmount}`);
}
function function_type_in_args_ex2_test() {
    // Function Type: (number) => (number)
    var usTaxCalculator = (value: number) => {
        return value * 10 / 100;
    };
    // Function Type: (number) => (number)
    var vnTaxCalculator: TaxCalculator = (value: number) => value * 5 / 100;

    printTaxAmount(1000, usTaxCalculator);
    printTaxAmount(1000, vnTaxCalculator);
    printTaxAmount(1000, (value: number) => value * 8 / 100);
}
function_type_in_args_ex2_test(); // Call the function.
Output:

Tax Amount: 100
Tax Amount: 50
Tax Amount: 80

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