save() {
    const arr = this.data.fullReduction;
    const fullArr = arr.map((item) => Number(item?.full));
    const dicountArr = arr.map((item) => Number(item?.discount));
    // const isLess = arr.every((item) => Number(item.full) > Number(item.discount));
    // const isSortedFull = fullArr.every((item, i) => i === 0 || item > fullArr[i - 1]);
    // const isSortedDiscount = dicountArr.every((item, i) => i === 0 || item > dicountArr[i - 1]);
    let fullNullIndex = arr.findIndex((item) => item.full === null || item.full === '');
    let discountNullIndex = arr.findIndex((item) => item.discount === null || item.discount === '');
    let notlessIndex = arr.findIndex((item) => Number(item.full) <= Number(item.discount));
    let notSortedFull = fullArr.findIndex((item, i) => (i === 0 || item > fullArr[i - 1]) === false);
    let notSortedDiscount = dicountArr.findIndex((item, i) => (i === 0 || item > dicountArr[i - 1]) === false);
    console.log('arr', arr);
    debugger;
    if (fullNullIndex !== -1) {
      wx.showToast({
        title: `第${notlessIndex + 1}项门槛金额尚未填写`,
        icon: 'none',
      });
    } else if (discountNullIndex !== -1) {
      wx.showToast({
        title: `第${notlessIndex + 1}项折扣金额尚未填写`,
        icon: 'none',
      });
    } else if (notlessIndex !== -1) {
      wx.showToast({
        title: `第${notlessIndex + 1}项门槛金额必须大于折扣金额`,
        icon: 'none',
      });
    } else if (notSortedFull !== -1) {
      wx.showToast({
        title: `门槛金额必须递增(第${notSortedFull + 1}项)`,
        icon: 'none',
      });
    } else if (notSortedDiscount !== -1) {
      wx.showToast({
        title: `高门槛金额必须大于低门槛折扣金额(第${notSortedDiscount + 1}项)`,
        icon: 'none',
      });
    }
  },

优化后：

validate() {
    // 校验是否通过，并给出提示信息
    // return true or false
    const arr = this.data.fullReduction;
    const fullArr = arr.map((item) => Number(item?.full));
    const dicountArr = arr.map((item) => Number(item?.discount));
    const validateConfig = [
      {
        key: 'fullNullIndex', // 金额尚未填写,
        toast: '项门槛金额尚未填写',
        fn: () => {
          return arr.findIndex((item) => item.full === null || item.full === '');
        },
      },
      {
        key: 'discountNullIndex',
        toast: '项折扣金额尚未填写',
        fn: () => {
          return arr.findIndex((item) => item.discount === null || item.discount === '');
        },
      },
      {
        key: 'notlessIndex',
        toast: '项门槛金额必须大于折扣金额',
        fn: () => {
          return arr.findIndex((item) => Number(item.full) <= Number(item.discount));
        },
      },
      {
        key: 'notSortedFull',
        toast: '门槛金额必须递增',
        fn: () => {
          return fullArr.findIndex((item, i) => (i === 0 || item > fullArr[i - 1]) === false);
        },
      },
      {
        key: 'notSortedDiscount',
        toast: '高门槛金额必须大于低门槛折扣金额',
        fn: () => {
          return dicountArr.findIndex((item, i) => (i === 0 || item > dicountArr[i - 1]) === false);
        },
      },
    ];

    for (let i = 0; i < validateConfig.length; i++) {
      const index = validateConfig[i].fn();
      if (index !== -1) {
        wx.showToast({
          title: `第${index + 1}项${validateConfig[i].toast}`,
          icon: 'none',
        });
        return false;
      }
    }
    return true;
  },

  save() {
    const result = this.validate();
  },