FontData: blob() method

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The blob() method of the FontData interface returns a Promise that fulfills with a Blob containing the raw bytes of the underlying font file.

Syntax

js
blob()

Parameters

None.

Return value

A Promise that fulfills with a Blob containing the raw bytes of the underlying font file.

Examples

The blob() method provides access to low-level SFNT data — this is a font file format that can contain other font formats, such as PostScript, TrueType, OpenType, or Web Open Font Format (WOFF).

js
async function computeOutlineFormat() {
  try {
    const availableFonts = await window.queryLocalFonts({
      postscriptNames: ["ComicSansMS"],
    });
    for (const fontData of availableFonts) {
      // `blob()` returns a Blob containing valid and complete
      // SFNT-wrapped font data.
      const sfnt = await fontData.blob();
      // Slice out only the bytes we need: the first 4 bytes are the SFNT
      // version info.
      // Spec: https://learn.microsoft.com/en-us/typography/opentype/spec/otff#organization-of-an-opentype-font
      const sfntVersion = await sfnt.slice(0, 4).text();

      let outlineFormat = "UNKNOWN";
      switch (sfntVersion) {
        case "\x00\x01\x00\x00":
        case "true":
        case "typ1":
          outlineFormat = "truetype";
          break;
        case "OTTO":
          outlineFormat = "cff";
          break;
      }
      console.log("Outline format:", outlineFormat);
    }
  } catch (err) {
    console.error(err.name, err.message);
  }
}

Specifications

Specification
Local Font Access
# ref-for-dom-fontdata-blob①

Browser compatibility

BCD tables only load in the browser

See also