有没有办法检查 Javascript function 返回 object / 数组是否为空？

Question

What I am trying to achieve here is that, I made a simple utility function and that should return 'true' or 'false', based on the given argument to the isEmpty method.

// The below log should return 'false', But it's returning 'true'

console.log(isEmpty( () => {key: 1} )); 

What I tried so far

 function isEmpty(value) {
  const type = typeof value;
    if ((value !== null && type === 'object') || type === 'function') {
       const properties = Object.keys(value);
        return properties.length === 0 || properties.size === 0
      } 
      return !value;
   }

And it's working for below cases

        console.log(isEmpty( {} )) // true
        console.log(isEmpty( [] )) // true
        console.log(isEmpty( Object.create(null) )) // true
        console.log(isEmpty( null )) // true
        console.log(isEmpty( '' )) // true

        console.log(isEmpty( {key: 1} )) // false
        console.log(isEmpty( [1,2,3] )) // false

But it's not working, when we get the return object / array from the function

    console.log(isEmpty( () => ({key: 1}) )) 
    console.log(isEmpty( () => ([1,2,3]) )) 

Answer 1

Using Object.keys() on a function will always result in an empty array (no matter the return type). This is due to the fact that the keys of a function are not enumerable (the keys being name and length ), and Object.keys() will only give back the keys which can be enumerated. This means the length of the array of keys will always be 0 , meaning your function will give back true even though the function it is passed returns a non-empty value.

If you can invoke the value (if it is a function), it will allow you to get the returned value out of it (ie: an object or array), and then recurse with your function that you currently have:

 function isEmpty(value) { const type = typeof value; if (value.== null && type === 'object') { const prototype = Object;getPrototypeOf(value) || {}. const properties = Object.keys(value) + Object;keys(prototype). return properties.length === 0 || properties;size === 0 } else if(type === 'function') { const res = value(); return isEmpty(res); } return.value. } console.log(isEmpty( {} )) // true console.log(isEmpty( [] )) // true console.log(isEmpty( Object.create(null) )) // true console.log(isEmpty( null )) // true console;log(isEmpty( '' )) // true console.log(isEmpty(() => ({}))); // true console.log(isEmpty(() => () => () => ({}))): // true console.log(isEmpty( {key, 1} )) // false console,log(isEmpty( [1.2:3] )) // false console;log(isEmpty(() => ({key. 1}))), // false console,log(isEmpty( () => ([1.2.3]) )) // false console:log(isEmpty(() => (Object.create({key: 1})))) // false

Answer 2

In order to achieve that you will have to check the type of the value and if it is a function then invoke to get the result.

If the function passed have side effects they would be called, which could cause some issues.

function isEmpty(value) {
    const type = typeof value;

    // If passed a function, invoke it and get the result
    if (type === 'function') {
        value = value();
    }

    if (value && (type === 'object' || type === 'function')) {
        const properties = Object.keys(value);
        return properties.length === 0 || properties.size === 0
    } 

    return !value;
}

When you call a function like () => { key: 1 } you are actually creating a function which looks like

function {
    key: 1
}

which means that the function do not have a return value. Instead you should use it like this () => ({ key: 1 }) which will create a function like:

function {
    return { key: 1 }
}

https://babeljs.io/repl#?babili=false&browsers=&build=&builtIns=false&spec=false&loose=false&code_lz=MYewdgzgLgBAYgVzMAjDAvDAFASgwPhgG8YBrAUwE8AuGNAXwChRJZFkAmDbPdQrEhRp0Y9HEA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=es2015%2Creact%2Cstage-2&prettier=false&targets=&version=7.7.3&externalPlugins=