Angular 2 pipe that transforms JSON object to pretty-printed JSON

I would like to add an even simpler way to do this, using the built-in json pipe:

<pre>{{data | json}}</pre>

This way, the formatting is preserved.

I would create a custom pipe for this:

@Pipe({
  name: 'prettyprint'
})
export class PrettyPrintPipe implements PipeTransform {
  transform(val) {
    return JSON.stringify(val, null, 2)
      .replace(' ', ' ')
      .replace('\n', '<br/>');
  }
}

and use it this way:

@Component({
  selector: 'my-app',
  template: `
    <div [innerHTML]="obj | prettyprint"></div>
  `,
  pipes: [ PrettyPrintPipe ]
})
export class AppComponent {
  obj = {
    test: 'testttt',
    name: 'nameeee'
  }
}

See this stackblitz: https://stackblitz.com/edit/angular-prettyprint

I had required this scenario and many times require it. I saw this question is still trending in 2021. So I created a detailed post explaining not how to just prettify it but add colors to it and built a small tool to play around with.

2021 solution: I built my own custom version of pipe (inspried by this answer) which not only prettifies but also adds colors to JSON like vscode. I don't use a built-in JSON pipe because it doesn't serve my full purpose.

This also gives you the freedom to add number lines and padding if you wish to. Try to play around even with the nested jsons!

Sample output like below

global stylesheet should contain colors as per your theme for e.g styles.scss

pre {
  font-weight: 400;

  .number-line {
    color: #adadaf;
  }
  .string {
    color: #95c602;
  }
  .number {
    color: #f2b619;
  }
  .boolean {
    color: #0097f1;
  }
  .null {
    color: #727990;
  }
  .key {
    color: #fff;
  }
}

Source code of the pipe

@Pipe({
  name: 'prettyjson',
  pure:true
})
export class PrettyJsonPipe implements PipeTransform {
  transform(value: any, args: any[]): any {
    try {
      /**
       * check and try to parse value if it's not an object
       * if it fails to parse which means it is an invalid JSON
       */
      return this.applyColors(
        typeof value === 'object' ? value : JSON.parse(value),
        args[0],
        args[1]
      );
    } catch (e) {
      return this.applyColors({ error: 'Invalid JSON' }, args[0], args[1]);
    }
  }

  applyColors(obj: any, showNumebrLine: boolean = false, padding: number = 4) {
    // line number start from 1
    let line = 1;

    if (typeof obj != 'string') {
      obj = JSON.stringify(obj, undefined, 3);
    }

    /**
     * Converts special charaters like &, <, > to equivalent HTML code of it
     */
    obj = obj.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;');
    /* taken from https://stackoverflow.com/a/7220510 */

    /**
     * wraps every datatype, key for e.g
     * numbers from json object to something like
     * <span class="number" > 234 </span>
     * this is why needed custom themeClass which we created in _global.css
     * @return final bunch of span tags after all conversion
     */
    obj = obj.replace(
      /("(\\u[a-zA-Z0-9]{4}|\\[^u]|[^\\"])*"(\s*:)?|\b(true|false|null)\b|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?)/g,
      (match: any) => {
        // class to be applied inside pre tag
        let themeClass = 'number';
        if (/^"/.test(match)) {
          if (/:$/.test(match)) {
            themeClass = 'key';
          } else {
            themeClass = 'string';
          }
        } else if (/true|false/.test(match)) {
          themeClass = 'boolean';
        } else if (/null/.test(match)) {
          themeClass = 'null';
        }
        return '<span class="' + themeClass + '">' + match + '</span>';
      }
    );

    /**
     * Regex for the start of the line, insert a number-line themeClass tag before each line
     */
    return showNumebrLine
      ? obj.replace(
          /^/gm,
          () =>
            `<span class="number-line pl-3 select-none" >${String(line++).padEnd(padding)}</span>`
        )
      : obj;
  }
}

now pass these params inside HTML like this. If you don't pass it by default value of showNumberline is false and padding is 4

<pre [innerHTML]="dummyJsonObject | prettyjson: [true, 3]"></pre>

Hope this helps🙂

As this is the first result on google, let me add a quick sum up:

• if you only need to print JSON without proper formatting, the build-in json pipe suggested by Shane Hsu works perfectly: <pre>{{data | json}}</pre>

• however, if you want to have a different output, you will need to create your own pipe as Thierry Templier suggested:

  1. ng g generate pipe prettyjson
  2. in prettyjson.pipe.ts:

import { Pipe, PipeTransform } from '@angular/core';

@Pipe({
  name: 'prettyjson'
})
export class PrettyjsonPipe implements PipeTransform {

  transform(value: any, ...args: any[]): any {
    return JSON.stringify(value, null, 2)
    .replace(/ /g, ' ') // note the usage of `/ /g` instead of `' '` in order to replace all occurences
    .replace(/\n/g, '<br/>'); // same here
  }

}

3. Finally, and because we return HTML content, the pipe must be used inside an innerHTML function:

<div [innerHTML]="data | prettyjson"></div>