赋值表达式类型字符串不能赋值为boolean类型

Question

我有点问题。我有一些数据和一些方法可以处理这些数据。但是当我想要更改属性固定时，我会收到错误。

export default {
    data() {
      return {
        classes: {
          titleSection: "title-section",
          fixed: false
        },
      }
    },
    methods: {
      handleScroll() {
        let coordsSection1 = this.getCoords(document.getElementById('section-1'));
        if (window.pageYOffset > coordsSection1) {
          this.classes.fixed = "fixed"; //this line get error
        }
        console.log(this.classes.fixed);
      },
      getCoords(elem) {
        let box = elem.getBoundingClientRect();
        return {
          top: box.top + pageYOffset,
          left: box.left + pageXOffset
        }
      }
    },
    created() {
      window.addEventListener('scroll', this.handleScroll);
    },
    destroyed() {
      window.removeEventListener('scroll', this.handleScroll);
    }
  }