isX functions, really helps to write less. Most common for me:
function isBoolean (v) { return typeof v === 'boolean'; }
function isNumber (v) { return typeof v === 'number'; }
function isString (v) { return typeof v === 'string'; }
function isNumeric (v) { return isNumber(v) || (isString(v) && String(v * 1) === v); }
function isSymbol (v) { return typeof v === 'symbol'; }
function isObject (v) { return v !== null && v === Object(v) && Object.prototype.toString.call(v) === '[object Object]'; }
function isFunction (v) { return v && typeof v === 'function'; }
function isArray (v) { return v && Array.isArray(v); }
function isElement (v) { return v && v.tagName; }
function isset (v) { return v !== null && typeof v !== 'undefined'; }