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

function createDependencyProposals() {
    // returning a static list of proposals, not even looking at the prefix (filtering is done by the Monaco editor),
    // here you could do a server side lookup
    return [
        {
            label: '"lodash"',
            kind: monaco.languages.CompletionItemKind.Function,
            documentation: "The Lodash library exported as Node.js modules.",
            insertText: '"lodash": "*"'
        },
        {
            label: '"express"',
            kind: monaco.languages.CompletionItemKind.Function,
            documentation: "Fast, unopinionated, minimalist web framework",
            insertText: '"express": "*"'
        },
        {
            label: '"mkdirp"',
            kind: monaco.languages.CompletionItemKind.Function,
            documentation: "Recursively mkdir, like <code>mkdir -p</code>",
            insertText: '"mkdirp": "*"'
        }
    ];
}


monaco.languages.registerCompletionItemProvider('json', {
    provideCompletionItems: function(model, position) {
        // find out if we are completing a property in the 'dependencies' object.
        var textUntilPosition = model.getValueInRange({startLineNumber: 1, startColumn: 1, endLineNumber: position.lineNumber, endColumn: position.column});
        var match = textUntilPosition.match(/"dependencies"\s*:\s*{\s*("[^"]*"\s*:\s*"[^"]*"\s*,\s*)*("[^"]*)?$/);
        if (match) {
            return createDependencyProposals();
        }
        return [];
    }
});

monaco.editor.create(document.getElementById("container"), {
    value: "{\n\t\"dependencies\": {\n\t\t\n\t}\n}\n",
    language: "json"
});