Modern JavaScript: variable declaration

June 28, 2020

In addition to var, a variable can now be declared using let and const keywords.

There are some details to be aware in order to use them properly. Let see some examples.

var

var is function-scoped or globally-scoped. For example:

function testVar() {
    var test = "Hello World!";

    if (true) {
        var test = "Something else";
        console.log(test); // expected output: Something else
    }

    console.log(test); // expected output: Something else
}

testVar();

let

let is block-scoped. For example, we can replace let with var in the previous example. The output, as we can see, is different:

function testLet() {
    let test = "Hello World!";

    if (true) {
        let test = "Something else";
        console.log(test); // expected output: Something else
    }

    console.log(test); // expected output: Hello World!
}

testLet();

const

const works like let, but the value can't be reassigned or redeclared.

const testConst = "Hello";
testConst = "Hello World!"; // expected output: TypeError exception

const does not create immutable objects! Keep this always in mind. Even if we declare an object with const, we are still able to change is content:

const testConstObject = {a: "Hello World!"};
testConstObject.a = "Something else"; // no exception will be throw

To create an immutable object we can use Object.freeze(), but even in this case JavaScript does not provide a real immutable object: only the first level of the object is immutable, but all the others level can be changed through reassignement.

References

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