vant/src/nav-bar/index.js

// Utils
import { createNamespace } from '../utils';
import { BORDER_BOTTOM } from '../utils/constant';

// Components
import Icon from '../icon';

const [createComponent, bem] = createNamespace('nav-bar');

export default createComponent({
  props: {
    title: String,
    fixed: Boolean,
    zIndex: [Number, String],
    leftText: String,
    rightText: String,
    leftArrow: Boolean,
    placeholder: Boolean,
    safeAreaInsetTop: Boolean,
    border: {
      type: Boolean,
      default: true,
    },
  },

  data() {
    return {
      height: null,
    };
  },

  mounted() {
    if (this.placeholder && this.fixed) {
      const setHeight = () => {
        this.height = this.$refs.navBar.getBoundingClientRect().height;
      };

      setHeight();
      // https://github.com/vant-ui/vant/issues/10131
      setTimeout(setHeight, 100);
    }
  },

  methods: {
    genLeft() {
      const leftSlot = this.slots('left');

      if (leftSlot) {
        return leftSlot;
      }

      return [
        this.leftArrow && <Icon class={bem('arrow')} name="arrow-left" />,
        this.leftText && <span class={bem('text')}>{this.leftText}</span>,
      ];
    },

    genRight() {
      const rightSlot = this.slots('right');

      if (rightSlot) {
        return rightSlot;
      }

      if (this.rightText) {
        return <span class={bem('text')}>{this.rightText}</span>;
      }
    },

    genNavBar() {
      return (
        <div
          ref="navBar"
          style={{ zIndex: this.zIndex }}
          class={[
            bem({
              fixed: this.fixed,
              'safe-area-inset-top': this.safeAreaInsetTop,
            }),
            {
              [BORDER_BOTTOM]: this.border,
            },
          ]}
        >
          <div class={bem('content')}>
            {this.hasLeft() && (
              <div class={bem('left')} onClick={this.onClickLeft}>
                {this.genLeft()}
              </div>
            )}
            <div class={[bem('title'), 'van-ellipsis']}>
              {this.slots('title') || this.title}
            </div>
            {this.hasRight() && (
              <div class={bem('right')} onClick={this.onClickRight}>
                {this.genRight()}
              </div>
            )}
          </div>
        </div>
      );
    },

    hasLeft() {
      return this.leftArrow || this.leftText || this.slots('left');
    },

    hasRight() {
      return this.rightText || this.slots('right');
    },

    onClickLeft(event) {
      this.$emit('click-left', event);
    },

    onClickRight(event) {
      this.$emit('click-right', event);
    },
  },

  render() {
    if (this.placeholder && this.fixed) {
      return (
        <div class={bem('placeholder')} style={{ height: `${this.height}px` }}>
          {this.genNavBar()}
        </div>
      );
    }

    return this.genNavBar();
  },
});