monaco-editor/website/playground/new-samples/extending-language-services/model-markers-example/sample.js

function validate(model) {
	const markers = [];
	// lines start at 1
	for (let i = 1; i < model.getLineCount() + 1; i++) {
		const range = {
			startLineNumber: i,
			startColumn: 1,
			endLineNumber: i,
			endColumn: model.getLineLength(i) + 1
		};
		const content = model.getValueInRange(range).trim();
		const number = Number(content);
		if (Number.isNaN(number)) {
			markers.push({
				message: 'not a number',
				severity: monaco.MarkerSeverity.Error,
				startLineNumber: range.startLineNumber,
				startColumn: range.startColumn,
				endLineNumber: range.endLineNumber,
				endColumn: range.endColumn
			});
		} else if (!Number.isInteger(number)) {
			markers.push({
				message: 'not an integer',
				severity: monaco.MarkerSeverity.Warning,
				startLineNumber: range.startLineNumber,
				startColumn: range.startColumn,
				endLineNumber: range.endLineNumber,
				endColumn: range.endColumn
			});
		}
	}
	monaco.editor.setModelMarkers(model, 'owner', markers);
}

const value = `12345
abcd
234.56
12345
abcd
234.56`;
const uri = monaco.Uri.parse('inmemory://test');
const model = monaco.editor.createModel(value, 'demoLanguage', uri);
editor = monaco.editor.create(document.getElementById('container'), { model });
validate(model);
model.onDidChangeContent(() => {
	validate(model);
});