先放效果：

 代码

<template>
  <el-input v-model="num" placeholder="请输入数字" />
  <el-button type="primary" @click="changeNum">取绝对值</el-button>
</template>

import { ref } from "vue";

const num = ref();
const changeNum = () => {
  if (num.value) {
    num.value = Math.abs(num.value);
  }
};