When writing a function, we find that el-select supports multiple elections, but it doesn't support all elections. It's wordless. Let's do it by ourselves. There are two ways. The second way is simpler.

Method 1: Add a drop-down item, and then there should be the following situations:

1. When all drop-down options are checked, [full selection] is automatically checked.
2. When the drop-down option part is checked, all drop-down items are checked after clicking on the full selection.
3. When all the drop-down options are not checked, click on the [full selection], all the drop-down options are not checked.
4. When the drop-down option and the [full selection] are both selected, the [full selection] button will not be checked unless any drop-down option is checked.

The above is what I want to achieve, I'm very verbose... Look at the code...

html section:

<template>
    <el-select multiple collapse-tags v-model='selectedArray' @change='changeSelect' @remove-tag='removeTag' placeholder='Please choose'>
        <el-option label='All election' value='All election' @click.native='selectAll'></el-option>
        <el-option v-for='(item, index) in options' :key='index' :label='item.name' :value='item.name'></el-option>
     </el-select>
</template>

Part js:

export default {
  data() {
    return {
      selectedArray: [],
      options: [
        { name: 'one by one', label: 'one' },
        { name: '22', label: 'tow' },
        { name: '33', label: 'three' },
        { name: '44', label: 'four' },
        { name: '55', label: 'five' }
      ]
    }
  },
  methods: {
    selectAll() {
      if (this.selectedArray.length < this.options.length) {
        this.selectedArray = []
        this.options.map((item) => {
          this.selectedArray.push(item.name)
        })
        this.selectedArray.unshift('All election')
      } else {
        this.selectedArray = []
      }
    },
    changeSelect(val) {
      if (!val.includes('All election') && val.length === this.options.length) {
        this.selectedArray.unshift('All election')
      } else if (val.includes('All election') && (val.length - 1) < this.options.length) {
        this.selectedArray = this.selectedArray.filter((item) => {
          return item !== 'All election'
        })
      }
    },
    removeTag(val) {
      if (val === 'All election') {
        this.selectedArray = []
      }
    }
  }
}

Look at the picture.

Method 2: Add a check box directly to achieve the same function as method 1.
html section:

<template>
  <el-select multiple collapse-tags v-model='selectedArray' @change='changeSelect' placeholder='Please choose'>
    <el-checkbox v-model="checked" @change='selectAll'>All election</el-checkbox>
    <el-option v-for='(item, index) in options' :key='index' :label='item.name' :value='item.name'></el-option>
  </el-select>
</template>

Part js:

export default {
  data() {
    return {
      checked: false,
      selectedArray: [],
      options: [
        { name: 'one by one', label: 'one' },
        { name: '22', label: 'tow' },
        { name: '33', label: 'three' },
        { name: '44', label: 'four' },
        { name: '55', label: 'five' }
      ]
    }
  },
  methods: {
    selectAll() {
      this.selectedArray = []
      if (this.checked) {
        this.options.map((item) => {
          this.selectedArray.push(item.name)
        })
      } else {
        this.selectedArray = []
      }
    },
    changeSelect(val) {
      if (val.length === this.options.length) {
        this.checked = true
      } else {
        this.checked = false
      }
    }
  }
}

css:

.el-checkbox {
    text-align: right;
    width: 100%;
    padding-right: 10px;
  }

Design sketch: