NeTT•13mo ago

Custom console.log formatting for a class/object

Is there a way to change the way an object/class is printed in the console? For example, instead of

[1, 2, 3, 4]

[1, 2, 3, 4]

, I want the particular object to be printed as

[1, 2
 3, 4]

[1, 2
 3, 4]

5 Replies

ybabts•13mo ago

see Deno.inspect https://deno.land/api@v1.35.1?s=Deno.inspect

ybabts•13mo ago

you can specify some options for how you want the object to be displayed https://deno.land/api@v1.35.1?s=Deno.InspectOptions

Deno

Deno.InspectOptions | Runtime APIs | Deno

Option which can be specified when performing {@linkcode Deno.inspect}.

ybabts•13mo ago

if you want to have a more specific way of displaying your object, you can use the Deno.customInspect symbol to implement your own way of inspecting the object https://deno.land/manual@v1.35.1/basics/testing/snapshot_testing#serialization-with--denocustominspect

Deno

Snapshots | Manual | Deno

The Deno standard library comes with a snapshot module, which enables developers to write tests which assert a value against a reference snap

ybabts•13mo ago

Here's a little example it showing off

const myarray = [1, 2, 3, 4, 5, 6];

console.log(myarray);
console.log(Deno.inspect(myarray, {
  breakLength: 28,
}));
Object.defineProperty(myarray, Symbol.for("Deno.customInspect"), {
  value() {
    const splits = String(this).split(",");
    let str = "";
    for (let i = 0; i < splits.length; i += 2) {
      str += (i !== 0 ? " " : "") + (splits[i] + ", " + splits[i + 1] ?? "") +
        (i > splits.length / 2 ? "" : "\n");
    }
    return `[${str}]`;
  },
});
console.log(myarray);

const myarray = [1, 2, 3, 4, 5, 6];

console.log(myarray);
console.log(Deno.inspect(myarray, {
  breakLength: 28,
}));
Object.defineProperty(myarray, Symbol.for("Deno.customInspect"), {
  value() {
    const splits = String(this).split(",");
    let str = "";
    for (let i = 0; i < splits.length; i += 2) {
      str += (i !== 0 ? " " : "") + (splits[i] + ", " + splits[i + 1] ?? "") +
        (i > splits.length / 2 ? "" : "\n");
    }
    return `[${str}]`;
  },
});
console.log(myarray);

stdout

[ 1, 2, 3, 4, 5, 6 ]
[
  1,
  2,
  3,
  4,
  5,
  6
]
[1, 2
 3, 4
 5, 6]

[ 1, 2, 3, 4, 5, 6 ]
[
  1,
  2,
  3,
  4,
  5,
  6
]
[1, 2
 3, 4
 5, 6]

you can also define it for specific object classes as well if you want, or implement the symbol in your own class

Object.defineProperty(Array.prototype, Symbol.for("Deno.customInspect"), { ... })

Object.defineProperty(Array.prototype, Symbol.for("Deno.customInspect"), { ... })

NeTT•13mo ago

Ooh thank you I'll take a look into it