demo/typescript.js
function getSignatureHelpItems(fileName, position) {
            synchronizeHostData();
            var sourceFile = getValidSourceFile(fileName);
            return ts.SignatureHelp.getSignatureHelpItems(program, sourceFile, position, cancellationToken);
        }
function getSignatureHelpItems(fileName, position) {
            synchronizeHostData();
            var sourceFile = getValidSourceFile(fileName);
            return ts.SignatureHelp.getSignatureHelpItems(program, sourceFile, position, cancellationToken);
        }
Signature help 
This is a semantic operation.