Get iteration number in for...of loop while iterating over an object JavaScript [duplicate]

Question

I am getting the following object from API call:

{
    "email": ["The email has already been taken."],
    "profile_image": [ "profile image is requried." ]
}

I used the following code to iterate over this object (using for...of):

let message = "";
for (const [error_name, error_value, i] of Object.entries(
  error.response.data.errors
)) {
  message += `${i}: ${error_value[0]}\n`;
}

But I am getting undefined instead of getting the value of i. I want to get the iteration number.

Any suggestions?

Object.entries returns a 2d array, where the inner arrays represent key-value pairs ([key, value]), so you can't destructure a value i which isn't in this inner-array — Nick Parsons
– Nick Parsons, Commented Jul 31, 2019 at 11:51
Why are you not just using forEach or map or even better reduce? — epascarello
– epascarello, Commented Jul 31, 2019 at 11:51
for const [index, [error_name, error_value]] of Object.entries( error.response.data.errors ).entries()) — Jonas Wilms
– Jonas Wilms, Commented Jul 31, 2019 at 12:06

CertainPerformance · Accepted Answer · 2019-07-31 11:50:53Z

4

An Object.entries item will only contain two values: the key and the value. If you want the index as well, you'll have to implement it yourself or use another strategy. One option is use forEach to iterate over the array instead (the second argument forEach takes is the current index):

let message = "";
Object.entries(error.response.data.errors)
  .forEach(([error_name, error_value], i) => {
    message += `${i}: ${error_value[0]}\n`;
  });

You can also avoid the let for message by using .map/.join:

const message = Object.entries(error.response.data.errors)
  .map(([error_name, error_value], i) => `${i}: ${error_value[0]}`)
  .join('\n');

(unlike your original code, this will not have a trailing newline at the end, but that's probably a good thing)

answered Jul 31, 2019 at 11:50

CertainPerformance

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4 Comments

ok. Over a year ago

Bro can you tell me if there is only 1 element in the object then don't display ${i}: part.

CertainPerformance Over a year ago

I'd save the entries in a variable and use the conditional operator. const message = errorEntries.length === 1 ? errorEntries[1] : entries.map(...code in answer...)

ok. Over a year ago

Bro the 3rd param is the obj which get passed I wrote,

const message = Object.entries(error.response.data.errors)                 .map(([error_name, error_value], index, obj) =>                   Object.keys(obj).length == 1                     ? ${error_value[0]}                     : ${index + 1}: ${error_value[0]}                 )                 .join("\n");

CertainPerformance Over a year ago

Much better to save the entries in a variable first, rather than to try to stuff it all into a single assignment expression

T.J. Crowder · Accepted Answer · 2019-07-31 11:57:47Z

The iterator from Object.entries only produces two-place arrays in the form [key, value]. There is no third entry in those arrays.

In general, objects should be considered unordered, so it's not clear why you want an index. But if you do, since you're building a string with concatenation, you could use map on the Object.entries array instead:

let message =
    Object.entries(error.response.data.errors)
    .map(([error_name, error_value], i) => `${i}: ${error_value[0]}\n`)
    .join("");

Live Example:

const error = {
    response: {
        data: {
            errors: {
              "email": ["The email has already been taken."],
              "profile_image": [ "profile image is requried." ]
            }
        }
    }
};
let message =
    Object.entries(error.response.data.errors)
    .map(([error_name, error_value], i) => `${i}: ${error_value[0]}\n`)
    .join("");
console.log(message);

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