FontFaceSet.check()
  The check() method of the FontFaceSet returns whether all
  fonts in the given font list have been loaded and are available.
Syntax
aFontFaceSet.check(font);
aFontFaceSet.check(font, text);
Returns
A Boolean value that is true if the font list is available.
Parameters
font- 
    
a font specification using the CSS value syntax, for example
"italic bold 16px Roboto" text- 
    
limit the font faces to those whose Unicode range contains at least one of the characters in text. This does not check for individual glyph coverage.
 
Examples
In the following example, the first line will print true if the Courier font is available at 12px. The second line will print true if the font MyFont contains the "ß" character.
console.log(document.fonts.check("12px courier"));
console.log(document.fonts.check("12px MyFont", "ß"));
Specifications
| Specification | 
|---|
| CSS Font Loading Module Level 3  # dom-fontfaceset-check  | 
Browser compatibility
BCD tables only load in the browser