Unexpected outcome when returning a map

Question

Unexpected outcome when returning a map

Encountered a puzzling issue that requires immediate clarification. When I input the following code into my project:

this.metadata = json.metadata.map((x) => {return new Metadatum(x);});
console.log(this.metadata[0].value);

The output consistently shows 'undefined' for every element in json.metadata.

However, when I switch gears and use the map function incorrectly (I acknowledge that using forEach would be more appropriate), like so:

json.metadata.map(x => this.metadata.push(new Metadatum(x)));
console.log(this.metadata[0].value);

Suddenly, the correct values are displayed instead of undefined.

I am left puzzled as to why assigning to this.metadata (which is an array of Metadatum objects) results in undefined.

javascript typescript

Answer 1

Answer №1

Make sure to review your data structure; the .map function takes an array and then creates a new one based on it. Check out this example:

var keyValueArray = [
    {name:'Alice', score:90},
    {name:'Bob', score:80}, 
    {name:'Charlie', score: 70}
];

var transformedArray = keyValueArray.map(function(item){ 
    var newItem = {};
    newItem[item.name] = item.score;
    return newItem;
});

Answer 2

Make sure to review your data structure; the .map function takes an array and then creates a new one based on it. Check out this example:

var keyValueArray = [
    {name:'Alice', score:90},
    {name:'Bob', score:80}, 
    {name:'Charlie', score: 70}
];

var transformedArray = keyValueArray.map(function(item){ 
    var newItem = {};
    newItem[item.name] = item.score;
    return newItem;
});

Answer 3

Answer №2

If you want to utilize the .map method, there are several options available:

var data = [1, 2,3,4,5,6,7];
function Metadatum (x) {
  this.value = x;
}

let metadata = data.map((x) => { return new Metadatum(x); });
let metadata1 = data.map(x => new Metadatum(x));
console.log(metadata[0].value);
console.log(metadata1[0].value);

Here are some examples of ES6 arrow function syntax:

(param1, param2, …, paramN) => { statements }
(param1, param2, …, paramN) => expression
     // equivalent to:  => { return expression; }

// Parentheses are optional when there's only one parameter:
(singleParam) => { statements }

singleParam => { statements }

// A function with no parameters requires parentheses:
() => { statements }

To learn more about arrow functions and their syntax in detail, you can visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions

EDIT: To see a working example, check out this JSBin link: https://jsbin.com/huzuyo/1/edit?html,js,console

Answer 4

If you want to utilize the .map method, there are several options available:

var data = [1, 2,3,4,5,6,7];
function Metadatum (x) {
  this.value = x;
}

let metadata = data.map((x) => { return new Metadatum(x); });
let metadata1 = data.map(x => new Metadatum(x));
console.log(metadata[0].value);
console.log(metadata1[0].value);

Here are some examples of ES6 arrow function syntax:

(param1, param2, …, paramN) => { statements }
(param1, param2, …, paramN) => expression
     // equivalent to:  => { return expression; }

// Parentheses are optional when there's only one parameter:
(singleParam) => { statements }

singleParam => { statements }

// A function with no parameters requires parentheses:
() => { statements }

To learn more about arrow functions and their syntax in detail, you can visit: https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Functions/Arrow_functions

EDIT: To see a working example, check out this JSBin link: https://jsbin.com/huzuyo/1/edit?html,js,console

Unexpected outcome when returning a map

Answer №1

Answer №2

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