traverse

1284 bytes

Usage

Recursively visit each property of an object (or each element of an array) and its nested objects or arrays. To traverse non-array iterables (e.g. Map, Set) and class instances, see the Traversing other objects section.

Traversal is performed in a depth-first manner. That means the deepest object will be visited before the last property of the root object.

import * as _ from 'radashi'

const root = { a: { b: 2 }, c: [1, 2] }

_.traverse(root, (value, key, parent, context) => {
  const depth = context.parents.length
  console.log(' '.repeat(depth * 2), key, '=>', value)
})
// Logs the following:
//   a => { b: 2 }
//     b => 2
//   c => [1, 2]
//     0 => 1
//     1 => 2

Tip: Check out the Advanced section to see what else is possible.

Did you know?

• Sparse arrays don’t have their holes visited.
• Circular references are skipped.

Types

TraverseVisitor

The TraverseVisitor type represents the function passed to traverse as its 2nd argument. If you ever need to declare a visitor separate from a traverse call, you can do so by declaring a function with this type signature.

import { TraverseVisitor } from 'radashi'

const visitor: TraverseVisitor = (value, key, parent, context) => {
  // ...
}

TraverseContext

Every visit includes a context object typed with TraverseContext, which contains the following properties:

• key: The current key being visited.
• parent: The parent object of the current value.
• parents: An array of objects (from parent to child) that the current value is contained by.
• path: An array describing the key path to the current value from the root.
• skip: A function used for skipping traversal of an object. If no object is provided, the current value is skipped. See Skipping objects for more details.
• skipped: A set of objects that have been skipped.
• value: The current value being visited.

Danger

The path and parents arrays are mutated by the traverse function. If you need to use them after the current visit, you should make a copy.

TraverseOptions

You may set these options for traverse using an object as its 3rd argument.

• ownKeys: A function that returns the own enumerable property names of an object.
• rootNeedsVisit: A boolean indicating whether the root object should be visited.

See the Options section for more details.

Options

Traversing all properties

By default, non-enumerable properties and symbol properties are skipped. You can pass in a custom ownKeys implementation to control which object properties are visited.

This example shows how Reflect.ownKeys can be used to include non-enumerable properties and symbol properties. Note that symbol properties are always traversed last when using Reflect.ownKeys.

import * as _ from 'radashi'

const symbol = Symbol('b')
const root = { [symbol]: 1 }
Object.defineProperty(root, 'a', { value: 2, enumerable: false })

_.traverse(
  root,
  (value, key) => {
    console.log(key, '=>', value)
  },
  { ownKeys: Reflect.ownKeys },
)
// Logs the following:
//   a => 2
//   Symbol(b) => 1

Visiting the root object

By default, your visitor callback will never receive the object passed into traverse. To override this behavior, set the rootNeedsVisit option to true.

When the root object is visited, the key will be null.

import * as _ from 'radashi'

const root = { a: 1 }

_.traverse(
  root,
  (value, key) => {
    console.log(key, '=>', value)
  },
  { rootNeedsVisit: true },
)
// Logs the following:
//   null => { a: 1 }
//   a => 1

Advanced

Traversing other objects

If traversing plain objects and arrays isn’t enough, try calling traverse from within another traverse callback like follows. This takes advantage of the fact that the root object is always traversed.

import * as _ from 'radashi'

// Note how we're using a named visitor function so it can reference itself.
_.traverse(root, function visitor(value, key, parent, context, options) {
  if (value instanceof MyClass) {
    return _.traverse(value, visitor, options, context)
  }
  // TODO: Handle other values as needed.
})

If you didn’t set any options, the options argument can be null:

return _.traverse(root, visitor, null, context)

Skipping objects

Using the TraverseContext::skip method, you can prevent an object from being traversed. By calling skip() with no arguments, the current value won’t be traversed.

import * as _ from 'radashi'

const root = {
  a: { b: 1 },
  c: { d: 2 },
}

_.traverse(root, (value, key, parent, context) => {
  console.log(key, '=>', value)

  // Skip traversal of the 'a' object.
  if (key === 'a') {
    context.skip()
  }
})
// Logs the following:
//   a => { b: 1 }
//   c => { d: 2 }
//   d => 2

You can pass any object to skip() to skip traversal of that object.

import * as _ from 'radashi'

const root = {
  a: {
    b: {
      c: 1,
    },
  },
}

_.traverse(root, (value, key, parent, context) => {
  console.log(key, '=>', value)

  // Visit the properties of the current object, but skip any objects nested within.
  Object.values(value).forEach(nestedValue => {
    if (_.isObject(nestedValue)) {
      context.skip(nestedValue)
    }
  })
})
// Logs the following:
//   a => { b: { c: 1 } }
//   b => { c: 1 }

Exiting early

If your visitor callback returns false, traverse will exit early and also return false. This is useful if you found what you wanted, so you don’t need to traverse the rest of the objects.

import * as _ from 'radashi'

let found = null
_.traverse(root, value => {
  if (isWhatImLookingFor(value)) {
    found = value
    return false
  }
})

Leave callbacks

If your visitor callback returns a function, it will be called once traverse has visited every visitable property/element within the current object. This is known as a “leave callback”.

Your leave callback can return false to exit traversal early.

import * as _ from 'radashi'

_.traverse({ arr: ['a', 'b'] }, (value, key) => {
  if (isArray(value)) {
    console.log('start of array')
    return () => {
      console.log('end of array')
      return false
    }
  } else {
    console.log(key, '=>', value)
  }
})
// Logs the following:
//     start of array
//     0 => 'a'
//     1 => 'b'
//     end of array