vue/src/core/vdom/helpers/normalize-children.js

/* @flow */

import VNode, { createTextVNode } from 'core/vdom/vnode'
import { isFalse, isDef, isUndef, isPrimitive } from 'shared/util'

// The template compiler attempts to minimize the need for normalization by
// statically analyzing the template at compile time.
//
// For plain HTML markup, normalization can be completely skipped because the
// generated render function is guaranteed to return Array<VNode>. There are
// two cases where extra normalization is needed:

// 1. When the children contains components - because a functional component
// may return an Array instead of a single root. In this case, just a simple
// normalization is needed - if any child is an Array, we flatten the whole
// thing with Array.prototype.concat. It is guaranteed to be only 1-level deep
// because functional components already normalize their own children.
export function simpleNormalizeChildren (children: any) {
  for (let i = 0; i < children.length; i++) {
    if (Array.isArray(children[i])) {
      return Array.prototype.concat.apply([], children)
    }
  }
  return children
}

// 2. When the children contains constructs that always generated nested Arrays,
// e.g. <template>, <slot>, v-for, or when the children is provided by user
// with hand-written render functions / JSX. In such cases a full normalization
// is needed to cater to all possible types of children values.
export function normalizeChildren (children: any): ?Array<VNode> {
  return isPrimitive(children)
    ? [createTextVNode(children)]
    : Array.isArray(children)
      ? normalizeArrayChildren(children)
      : undefined
}

function normalizeArrayChildren (children: any, nestedIndex?: string): Array<VNode> {
  const res = []
  let i, c, last
  for (i = 0; i < children.length; i++) {
    c = children[i]
    if (isUndef(c) || typeof c === 'boolean') continue
    last = res[res.length - 1]
    //  nested
    if (Array.isArray(c)) {
      res.push.apply(res, normalizeArrayChildren(c, `${nestedIndex || ''}_${i}`))
    } else if (isPrimitive(c)) {
      if (isDef(last) && isDef(last.text)) {
        last.text += String(c)
      } else if (c !== '') {
        // convert primitive to vnode
        res.push(createTextVNode(c))
      }
    } else {
      if (isFalse(c.isComment) && isDef(c.text) && isDef(last) && isFalse(last.isComment) && isDef(last.text)) {
        res[res.length - 1] = createTextVNode(last.text + c.text)
      } else {
        // default key for nested array children (likely generated by v-for)
        if (isDef(c.tag) && isUndef(c.key) && isDef(nestedIndex)) {
          c.key = `__vlist${nestedIndex}_${i}__`
        }
        res.push(c)
      }
    }
  }
  return res
}