TypeScript let

Introduction to TypeScript let

Variables in TypeScript can be declared using the keywords var, const, and let but since there are a few problems while using the keyword var and to overcome the problems posed while using var. We make use of the keyword let for variable declaration in TypeScript whose declaration is the same as the var declaration but the variables declared using let have block scope that is their scope is limited to the block within which they are declared and these variables cannot be accessed or modified outside the block they are declared and such variables declared using let keyword cannot be read or written to before they are declared in the program.

The syntax to declare variables using let in TypeScript is as follows:

let variable_name;

where variable_name is the name of the variable declared using let keyword.

Working of let keyword to declare variables in TypeScript is as follows:

• The variables in TypeScript can be declared using three keywords namely var, let, and const.
• There are several drawbacks while dealing with the variables declared using the var keyword.
• In order to overcome the drawbacks caused by declaring the variables using the var keyword, we use the let keyword to declare the variables.
• The variables declared using the let keyword have block scope that is the scope of these variables is limited to the blocks in which they are declared.
• The variables declared using the let keyword cannot be accessed or modified outside the block in which they are declared.
• The advantage of using the let keyword over var keyword to declare the variables is that the variables declared using the let keyword cannot be read or written to before they are declared in the program.

Examples of TypeScript let

Let us discuss examples of TypeScript let.

Example #1

TypeScript program to declare a variable using let keyword and to declare another variable using var keyword and demonstrate the difference of scope between the two variables:

Code:

//declaring a variable using var keyword
var globvar = 2;
//defining a function within which two variables num and num1 are declared using let keyword
function power()
{
let num = 10;
let num1 = Math.pow(num,globvar);
//defining a if condition statement within which a variable num2 is declared using let keyword
if(num1 >= 100)
{
let num2 = Math.sqrt(num1);
}
//displaying the value of the variables declared using var and let keywords within the scope of function
console.log("The value of the variable declared using var keyword is:\n",globvar);
console.log("The value of the variable num declared using let keyword inside the scope of function block is:\n",num);
console.log("The value of the variable num1 declared using let keyword inside the scope of function block is:\n",num1);
console.log("The value of the variable num2 declared using let keyword inside the scope of if block is:\n",num2);
}
power();
//displaying the value of the variables declared using var and let keywords outside the scope of function
console.log("The value of the variable declared using var keyword is:\n",globvar);
console.log("The value of the variable num declared using let keyword inside the scope of function block is:\n",num);
console.log("The value of the variable num1 declared using let keyword inside the scope of function block is:\n",num1);
console.log("The value of the variable num2 declared using let keyword inside the scope of if block is:\n",num2);

The output of the above program is as shown in the snapshot below:

The errors produced for the above program is as shown in the snapshot below:

In the above program, we are declaring a variable called globvar using the var keyword. Then we are declaring two other variables called num and num1 within the function using let keyword. Then within the if condition statement inside the function, we are defining another variable called num2 using the let keyword. Then we are displaying the value of variables declared within the scope of the function and hence the value of globvar, num, and num1 variables are displayed as the output on the screen. Because the variable num2 is declared inside the conditional statement block, we have received an error that num2 is declared but its value is never read and when we are trying to display the value of the variables outside the scope of function, the above errors have occurred as they are all declared using let keyword.

Example #2

TypeScript program to declare a variable using let keyword and to declare another variable using var keyword and demonstrate the difference of scope between the two variables:

Code:

//declaring a variable using var keyword
var globvar = 3;
//defining a function within which two variables num and num1 are declared using let keyword
function power()
{
let num = 2;
let num1 = Math.pow(num,globvar);
//defining a if condition statement within which a variable num2 is declared using let keyword
if(num1 >= 8)
{
let num2 = Math.sqrt(num1);
}
//displaying the value of the variables declared using var and let keywords within the scope of function
console.log("The value of the variable declared using var keyword is:\n",globvar);
console.log("The value of the variable num declared using let keyword inside the scope of function block is:\n",num);
console.log("The value of the variable num1 declared using let keyword inside the scope of function block is:\n",num1);
console.log("The value of the variable num2 declared using let keyword inside the scope of if block is:\n",num2);
}
power();
//displaying the value of the variables declared using var and let keywords outside the scope of function
console.log("The value of the variable declared using var keyword is:\n",globvar);
console.log("The value of the variable num declared using let keyword inside the scope of function block is:\n",num);
console.log("The value of the variable num1 declared using let keyword inside the scope of function block is:\n",num1);
console.log("The value of the variable num2 declared using let keyword inside the scope of if block is:\n",num2);

The output of the above program is as shown in the snapshot below:

The errors produced for the above program is as shown in the snapshot below:

In the above program, we are declaring a variable called globvar using var keyword. Then we are declaring two other variables called num and num1 within the function using let keyword. Then within the if condition statement inside the function, we are defining another variable called num2 using let keyword. Then we are displaying the value of variables declared within the scope of the function and hence the value of globvar, num, and num1 variables are displayed as the output on the screen. Because the variable num2 is declared inside the condition statement block, we have received an error that num2 is declared but its value is never read and when we are trying to display the value of the variables outside the scope of function, the above errors have occurred as they are all declared using let keyword.

Conclusion

In this article, we have learned the concept of let keyword in TypeScript through definition, syntax, and declaration of variables using let keyword through programming examples and their outputs.

This is a guide to TypeScript let. Here we discuss Definition, Working of let keyword to declare variables in TypeScript and examples with code implementation. You may also have a look at the following articles to learn more –

1. TypeScript Versions
2. TypeScript Array
3. Typescript Interface vs Class
4. TypeScript Types