The let and const Keywords
Learn how to define a variable's scope and constants using the let and const keywords in TypeScript.
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The let keyword
The fluid nature of JavaScript variables can sometimes cause errors when we inadvertently define variables with the same name but in a different scope within a code block.
Consider the following TypeScript code:
// Declare a variable called index with a type of number and assign it the value 0var index: number = 0;// If index is equal to 0, create a new block scope with a new variable also called index, but with a type of number and value of 2, and log its valueif (index == 0) {var index: number = 2;console.log(`index = ${index}`);}// Log the value of indexconsole.log(`index = ${index}`);
Variable index redeclared in the if-block
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On line 2, we define a variable named
indexof typenumberusing thevarkeyword and assign it a value of0. -
We then test if this value is equal to
0on line 5, and if it is, we enter a code block. -
The first statement in this code block on line 6 defines a variable named
indexof typenumberand ...