monaco-editor/website/playground/new-samples/extending-language-services/inlay-hints-provider-example/sample.js

const value = `
const f = (a, b) => a + b;

const result = f(2, 5);
`;

const editor = monaco.editor.create(document.getElementById("container"), {
    value,
    language: "javascript"
});

monaco.languages.registerInlayHintsProvider('javascript', {
    provideInlayHints(model, range, token) {
        return [{
            kind: monaco.languages.InlayHintKind.Type,
            position: { column: 13, lineNumber: 4 },
            text: `: Number`,
        }, {
            kind: monaco.languages.InlayHintKind.Type,
            position: { column: 13, lineNumber: 2 },
            text: `: Number`,
        }, {
            kind: monaco.languages.InlayHintKind.Type,
            position: { column: 16, lineNumber: 2 },
            text: `: Number`,
            whitespaceBefore: true, // see difference between a and b parameter
        }, {
            kind: monaco.languages.InlayHintKind.Parameter,
            position: { column: 18, lineNumber: 4 },
            text: `a:`,
        }, {
            kind: monaco.languages.InlayHintKind.Parameter,
            position: { column: 21, lineNumber: 4 },
            text: `b:`,
            whitespaceAfter: true, // similar to whitespaceBefore
        }]
    }
});