How can I use a variable as a field name in JSON stringify?

Question

I'd like to be able to dynamically created a JSON object using a variable as the field name.

I've worked out how to set the field value from a variable using $() but it doesn't work for the field name.

    const body: string = JSON.stringify({
      '__metadata': {
        'type': listItemEntityTypeName
      },
      `${FIELD_NAME}`: `${FIELD_VALUE}`
    });

The error I get in VS Code is:

[ts] Cannot invoke and expression whose type lacks a call signature. Type '{ '__metadata': { 'type':string; }; }' has no compatible call signatures. [2349] [ts] Property assignment expected. [1136]

I'm using typescript and react. I'm fairly new JavaScript so forgive me if I'm missing something obvious.

Possible duplicate of How to use a variable for a key in a JavaScript object literal? — Jeto
– Jeto, Commented Feb 3, 2019 at 9:49
Also, ${FIELD_VALUE} (with the backticks around) can probably just be FIELD_VALUE, unless you want to convert any value into a string. — Jeto
– Jeto, Commented Feb 3, 2019 at 9:51

Dhananjai Pai · Accepted Answer · 2019-02-03 09:52:50Z

8

Anything that goes inside JSON.stringify should be a valid JSON Object

const body: string = JSON.stringify({
  '__metadata': {
    'type': listItemEntityTypeName
  },
  [FIELD_NAME]: `${FIELD_VALUE}`   // FIELD_VALUE should also be fine, if you dont want to convert it to string.
});

removing the string literal in the key can give you a valid JSON and hoepfully resolve the error

edited Feb 3, 2019 at 9:52

answered Feb 3, 2019 at 9:50

Dhananjai Pai

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2 Comments

Jeto Over a year ago

FIELD_NAME will just be FIELD_NAME as a key here. He has a variable of the same name.

Dhananjai Pai Over a year ago

Good catch @Jeto

vmf91 · Accepted Answer · 2019-02-03 09:53:07Z

1

You can treat the object as an array.

var FIELD_NAME = 'hello'
var FIELD_VALUE = 'world'

var obj = {
    '__metadata': {
        'type': 'aaaa'
    },
}

obj[FIELD_NAME] = FIELD_VALUE

console.log('obj', obj)

answered Feb 3, 2019 at 9:53

vmf91

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Comments

hernanr · Accepted Answer · 2024-07-16 17:02:27Z

1

You can write your attribute name variable between square brackets, like this:

    const FIELD_NAME = 'Title';
    const FIELD_VALUE = 'New Title';
    const listItemEntityTypeName = 'Data.ListNameListItem';
    const body = JSON.stringify({
        __metadata: {
            type: listItemEntityTypeName,
        },
        [FIELD_NAME]: FIELD_VALUE,
    });
    
    console.log(body);

answered Jul 16, 2024 at 17:02

hernanr

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