Google闭包编译器抱怨typeof调用中的未声明变量

Question

I am minifying JavaScript for a typical modern browser (eg. Chrome 64) using Google's closure-compiler v20180101. The same code compiled without errors for v20170409. I am using the same piece of JS in multiple a several different packaging contexts and would rather not split things into many tiny files.

Instead, I've added some conditional code that uses typeof to see if some functions are already defined and if not, define them.

This is a very basic example:

if (typeof foo === 'undefined') {
  foo = function() { return "Foo!"; }
}

In response to this, clojure-compiler-v20180101 complains of:

foo.js: ERROR - variable foo is undeclared

I can add an extern file that declares var foo , but this seems to defeat the purpose of the typeof check. Also, in other situation foo gets declared twice which closure-compiler also complains about.

Thanks in advance for any assistance.

Answer 1

You need to declare the variable:

/** @suppress {duplicate} */
var foo;
if (typeof foo === 'undefined') {
  foo = function() { return "Foo!"; }
}