Block scope in JS

Both let and const are block scoped. They only exist within the innermost block that surrounds them.

function func() {
    if (true) {
        let tmp = 123;
    }
    console.log(tmp); // ReferenceError: tmp is not defined
}

In contrast, var variables are function scoped.

function func() {
    if (true) {
        var tmp = 123;
    }
    console.log(tmp); // 123
}

This is partly what leads to the classic loop closure bug. [[20210426174851-closure-loops-js]]

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