Javascript nested objects default fallback options

Question

I have a nested option object in javascript. What is the best way to provide default values for them?

Custom javascript object read from a json file with ajax:

{
  "fields": {
    "markdown": {
      "preview": {
        "delay": null,
        "custom": "hello"
      },
      "revisions": {
        "path": "revisions/markdown",
        "limit": 5
      }
    }
  }
}

Default options as fallback if values is not set:

{
  "fields": {
    "markdown": {
      "preview": {
        "delay": 5,
        "css": "https://example.com/my/style.css"
      },
      "revisions": {
        "path": "revisions",
        "limit": 10
      }
    }
  }
}

Expected results

Keep everything from custom object and complete it with default values, "css" in this case.

{
  "fields": {
    "markdown": {
      "preview": {
        "delay": null,
        "css": "https://example.com/my/style.css",
        "custom": "hello"
      },
      "revisions": {
        "path": "revisions/markdown",
        "limit": 5
      }
    }
  }
}

Notes:

• The custom object may not include all the nested values. If values are missing default value should apply.
• Null, empty and 0 is accepted values and should not be overwritten by default values.
• Values in the custom object that is missing as default values makes no harm and does not need to be dealt with.

What I've tried to far

• Spread operator does not merge nested objects - https://flaviocopes.com/how-to-merge-objects-javascript/
• Object assign does not merge nested objects - How can I merge properties of two JavaScript objects dynamically?

Answer 1

You could take a recursive approach by checking the value of the object, if an object and check the nested objects or assign the value if the key does not exist.

function update(object, fallback) {
    Object
        .entries(fallback)
        .forEach(([k, v]) => {
            if (v && typeof v === 'object') {
                update(object[k] = object[k] || (Array.isArray(v) ? [] : {}), v);
            } else if (!(k in object)) {
                object[k] = v;
            }
        });
}

var object = { fields: { markdown: { preview: { delay: null, custom: "hello" }, revisions: { path: "revisions/markdown", limit: 5 } } } },
    fallback = { fields: { markdown: { preview: { delay: 5, css: "https://example.com/my/style.css" }, revisions: { path: "revisions", limit: 10 } } } };

update(object, fallback);

console.log(object);

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