One sentence: The closure of a function is formed when it is defined, not when it is run.

Expand the syntactic sugar thoroughly and you should be able to see it more clearly

let foo = 'outer'; function fk_compiler() { return foo; } function bar(func) { if (func === undefined) { func = fk_compiler; } let foo = 'inner'; console.log(func()); } bar();

Look,fk_compilercan only returnfooin the external scope?

js is a lexical scope, and the value of foo takes the value when the function is defined rather than when it is executed.

Based on respondent code:

let foo = 'outer'; function fk_compiler() { return foo; } function bar(func) { if (func === undefined) { func = fk_compiler; } let foo = 'inner'; console.log(func()); } bar();

js adopts lexical scope, so no matter where the function is called, or in any form, its lexical scope is only determined by the position when it is declared.

fk_compileris declared in the global scope, so it will accessfooin the global scope. The answer will come out.

Similar code:

function foo(){ console.log(this.a); } (function init(){ var a = 'inner';//此处改为 window.a = 'global';再试试 foo(); })();