What are the differences between variables created using `let`, `var` or `const`?
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Variables declared using the var keyword are scoped to the function in which they are created, or if created outside of any function, to the global object. let and const are block scoped, meaning they are only accessible within the nearest set of curly braces (function, if-else block, or for-loop).
function foo() {// All variables are accessible within functions.var bar = 'bar';let baz = 'baz';const qux = 'qux';console.log(bar); // barconsole.log(baz); // bazconsole.log(qux); // qux}console.log(bar); // ReferenceError: bar is not definedconsole.log(baz); // ReferenceError: baz is not definedconsole.log(qux); // ReferenceError: qux is not defined
if (true) {var bar = 'bar';let baz = 'baz';const qux = 'qux';}// var declared variables are accessible anywhere in the function scope.console.log(bar); // bar// let and const defined variables are not accessible outside of the block they were defined in.console.log(baz); // ReferenceError: baz is not definedconsole.log(qux); // ReferenceError: qux is not defined
var allows variables to be hoisted, meaning they can be referenced in code before they are declared. let and const will not allow this, instead throwing an error.
console.log(foo); // undefinedvar foo = 'foo';console.log(baz); // ReferenceError: can't access lexical declaration 'baz' before initializationlet baz = 'baz';console.log(bar); // ReferenceError: can't access lexical declaration 'bar' before initializationconst bar = 'bar';
Redeclaring a variable with var will not throw an error, but let and const will.
var foo = 'foo';var foo = 'bar';console.log(foo); // "bar"let baz = 'baz';let baz = 'qux'; // Uncaught SyntaxError: Identifier 'baz' has already been declared
let and const differ in that let allows reassigning the variable's value while const does not.
// This is fine.let foo = 'foo';foo = 'bar';// This causes an exception.const baz = 'baz';baz = 'qux';