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Start your free trialMichael Cook
Full Stack JavaScript Techdegree Graduate 28,975 PointsConfused about block-level scoping
I am confused by block-level scoping. I thought I understood scope in JavaScript, but then I started taking the Intro to ES2015 course and realized I totally don't. Okay, so consider this example:
function foo() {
const x = 1;
if (someCondition) {
const x = 2;
console.log(x);
} else {
console.log(x);
}
}
In this example, a constant is declared in the top scope of the function, so it is accessible in all sub-blocks. But even though it is accessible in all sub-blocks, redefiningit in a sub-block is still legal. If the else
block runs, then 1
will be logged out. But if the if
branch is run, the constant will be redefined and 2
will be logged out. I just don't understand how sub-blocks can have access to a constant, but also be able to redefine a constant. This is turning my brain inside out a little. If anyone could help explain the scoping of let
and const
to me in an intuitive way I would be extremely grateful!
1 Answer
Steven Parker
231,169 PointsWhen you create a variable in a block with the same name as one in a larger scope it "shadows" the other one, meaning it makes the other one inaccessible. The original one is not reassigned, and in the case like this of a "const", reassigning would not be valid anyway.