循环中的Javascript对象属性不会删除

Question

I'm having trouble understanding the behavior of this javascript code.

const devices = searchResult.results.forEach(device => {
    const temp = Object.keys(device.fields);

    for(var property in temp) {
        if(device.fields.hasOwnProperty(property)) {
            if (!usedPropertiesAcrossModels.has(property)) {
                delete device.fields[property];
            }
        }
    }
}

I am trying to delete the keys if a javascript object that do not belong to a set. I have stepped through the debugger, and I know that there is only one element in the set and 15 elements in device.fields . No matter what, nothing is being deleted from device.fields , I have no idea why. Moreover, temp seems to be undefined until I am out of the loop. Property is always undefined even though there are items in temp! This doesn't make any sense.

Answer 1

 searchResult = {}; searchResult.results = [{ fields:{ name: 'hello', type:'gekko', random:'randomString' } } ] usedPropertiesAcrossModels = { name: 'hello', random:'hello' } const devices = searchResult.results.forEach(device => { const temp = Object.keys(device.fields).map((property)=>{ if(device.fields.hasOwnProperty(property)) { if (!usedPropertiesAcrossModels.hasOwnProperty(property)) { delete device.fields[property]; } } }) }) console.log(searchResult) 

Using map fixed the issue as in your case the for in was giving index instead of the keys of the object.Or as martin said you can consider using for of as well.

Answer 2

const temp = Object.keys(o) will give you array of object's keys. You should use for of loop instead of for in , as you need to iterate through values of it, not their keys in temp object:

 const o = { a: 1, b: 2, c: 3 }; const temp = Object.keys(o); console.log(temp); // this will iterate through `temp` keys, so 0, 1, 2 for (const property in temp) { console.log('wrong:', property); } // this will iterate through `temp` values, so 'a', 'b', 'c' for (const property of temp) { console.log('correct:', property); } // or you could iterate via `forEach()` temp.forEach(property => { console.log('correct:', property); }); 

Also with for of loop, you do not need the hasOwnProperty check.