Class array for loop fails

Question

I have this code that fails when I try to use the for loop

class TestA {
  testArray = [];
  constructor() {
    this.testArray.push({
      name: "joe",
      age: 70
    });
    this.testArray.push({
      name: "mike",
      age: 50
    });
    this.testArray.push({
      name: "bob",
      age: 33
    });
  }
  testLoop() {
    for (test of this.testArray) {
      console.log(" >>> " + test.name + " " + test.age);
    }
  }
}
var a = new TestA();
a.testLoop();

The error:

Uncaught ReferenceError: test is not defined

I can get away with that by using different kind of loop but don't understand why that doesn't work?

I'd point out that you don't need multiple, consecutive calls to .push(). Just add all the objects as arguments to a single call, and they all get added to the array. this.testArray.push({...}, {...}, {...}) — user12407908
– user12407908, Commented Jul 15, 2020 at 16:28
@Coder547: I think inside a class definition, you're running in strict mode, which sets more stringent requirements. Outside class you can manually put your code into that mode by using the "use strict" declarative. — user12407908
– user12407908, Commented Jul 15, 2020 at 16:38

Tu.Ma. · Accepted Answer · 2020-07-16 06:37:56Z

3

You forgot to declare test using let or const.

Use const if you want the identifier within the loop body to be read-only (so that, for instance, if someone modifies the code later to add an assignment, it's a proactive error in strict mode). Use let if you want to be able to assign to it (because you have an assignment in your code, or you want someone to be able to add one later without changing the declaration).

Extracted from: for...of loop. Should I use const or let?

class TestA {
  testArray = [];
  constructor() {
    this.testArray.push({
      name: "joe",
      age: 70
    });
    this.testArray.push({
      name: "mike",
      age: 50
    });
    this.testArray.push({
      name: "bob",
      age: 33
    });
  }
  testLoop() {
    for (let test of this.testArray) {
      console.log(" >>> " + test.name + " " + test.age);
    }
  }
}
var a = new TestA();
a.testLoop();

edited Jul 16, 2020 at 6:37

answered Jul 15, 2020 at 16:25

Tu.Ma.

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1 Comment

SGali Over a year ago

It would be good to clarify that he could also use the const keyword in this case

programmer22222 · Accepted Answer · 2020-07-15 16:25:01Z

2

You should write let for the line:

for (let test of this.testArray) {

answered Jul 15, 2020 at 16:25

programmer22222

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Comments

j08691 · Accepted Answer · 2020-07-15 16:25:34Z

1

In a for..of statement you need to declare your loop variable (test) with const, let, or var:

class TestA {
  testArray = [];
  constructor() {
    this.testArray.push({
      name: "joe",
      age: 70
    });
    this.testArray.push({
      name: "mike",
      age: 50
    });
    this.testArray.push({
      name: "bob",
      age: 33
    });
  }
  testLoop() {
    for (let test of this.testArray) {
      console.log(" >>> " + test.name + " " + test.age);
    }
  }
}
var a = new TestA();
a.testLoop();

answered Jul 15, 2020 at 16:25

j08691

208k33 gold badges269 silver badges281 bronze badges

Comments

Kunal Mukherjee · Accepted Answer · 2020-07-15 16:26:56Z

0

Bind the function in the constructor, as the this is not preserved.

class TestA {
  testArray = [];
  constructor() {
    this.testArray.push({
      name: "joe",
      age: 70
    });
    this.testArray.push({
      name: "mike",
      age: 50
    });
    this.testArray.push({
      name: "bob",
      age: 33
    });
    
    this.testLoop = this.testLoop.bind(this);
  }
  testLoop() {
    for (const test of this.testArray) {
      console.log(" >>> " + test.name + " " + test.age);
    }
  }
}
var a = new TestA();
a.testLoop();

answered Jul 15, 2020 at 16:26

Kunal Mukherjee

5,8733 gold badges29 silver badges55 bronze badges

4 Comments

Yury Tarabanko Over a year ago

This is completely unrelated and not necessary in the OP's case.

Kunal Mukherjee Over a year ago

@YuryTarabanko But he's trying to refer to this.testArray right, which will be undefined

Yury Tarabanko Over a year ago

It wont. He is using this method like a.testLoop() so it will have a as a context. You could try it. The only problem was that OP hadn't declared loop variable and class body is evaluated in strict mode.

user12407908 Over a year ago

If a.testLoop() didn't properly set this to the a object, that would make methods almost useless. You'd need to bind it if you're going to separate the method from the object before invoking it.

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