Will const and let make the IIFE pattern unnecessary?

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Yes, blocks are going to replace IEFEs, as soon as block-scoped declarations (functions, let/const/class) become widely adopted. You need a scope, e.g. for a closure? Here you have a block, be it a loop body or just part of a statement list.

However, there is still one application of IEFEs that blocks cannot replace: the module pattern. Blocks don’t have return values, and mutating higher-scoped variables is ugly, so we will still see function expressions in the creation of objects that need private state:

const example = (() => {
    …
    return …;
}());

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