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

function createDependencyProposals(range) {
	// 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": "*"',
			range: range
		},
		{
			label: '"express"',
			kind: monaco.languages.CompletionItemKind.Function,
			documentation: 'Fast, unopinionated, minimalist web framework',
			insertText: '"express": "*"',
			range: range
		},
		{
			label: '"mkdirp"',
			kind: monaco.languages.CompletionItemKind.Function,
			documentation: 'Recursively mkdir, like <code>mkdir -p</code>',
			insertText: '"mkdirp": "*"',
			range: range
		},
		{
			label: '"my-third-party-library"',
			kind: monaco.languages.CompletionItemKind.Function,
			documentation: 'Describe your library here',
			insertText: '"${1:my-third-party-library}": "${2:1.2.3}"',
			insertTextRules: monaco.languages.CompletionItemInsertTextRule.InsertAsSnippet,
			range: range
		}
	];
}

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 { suggestions: [] };
		}
		var word = model.getWordUntilPosition(position);
		var range = {
			startLineNumber: position.lineNumber,
			endLineNumber: position.lineNumber,
			startColumn: word.startColumn,
			endColumn: word.endColumn
		};
		return {
			suggestions: createDependencyProposals(range)
		};
	}
});

monaco.editor.create(document.getElementById('container'), {
	value: '{\n\t"dependencies": {\n\t\t\n\t}\n}\n',
	language: 'json'
});