不使用点表示法或括号表示法更新数组对象

Question

I would like to copy the size property of every object in dataArr to the corespondent size property of objects in testArr. I have problem to understand (and I'm surprised) why it is not working.

CODE:

 var testArr = [ {one : '1'}, {two : '2'}, {three : '3'} ]; var dataArr = [ {number : '0', size : '111'}, {number : '1', size : '222'}, {number : '2', size : '333'} ]; dataArr.forEach(function(item, index) { if (testArr[item.number] = item.number) { console.log('testArr[item.number] - ' + testArr[item.number]); // correct console.log('item.size - ' + item.size); // correct testArr[item.number].size = item.size; // not working testArr[item.number]['size'] = item.size; // not working either // WTF?! } }); console.log(testArr);

So why my testArr is not properly updated?

Answer 1

testArr[item.number] is an object like {one: '1'} , but item.number is a string. So they will never be equal, and the if test always fails.

But the reason you're getting into the if block is because you used = rather than == , so you're assigning instead of comparing.

You need to search testArr for the element whose value is the same as item.number .

 var testArr = [ {one : '1'}, {two : '2'}, {three : '3'} ]; var dataArr = [ {number : '0', size : '111'}, {number : '1', size : '222'}, {number : '2', size : '333'} ]; dataArr.forEach(function(item) { var obj = testArr.find(obj => Object.values(obj)[0] == item.number); if (obj) { obj.size = item.size; } }); console.log(testArr);

In general, an array of objects that each have a different key is usually poor design, because you can't access the property easily.

Answer 2

Like others have said. You have 2 errors in this line:

if (testArr[item.number] = item.number)

1. You are using the assignment operator = instead of equality operator == , and you should really use the type safe equality operator === to prevent erroneous type comparisons such as 0 == false returning true .
2. Even if you were using the correct operator, testArr contains objects, but you are comparing the object to a string item.number which doesn't make sense and will return false.