customElements.whenDefined()
customElements.whenDefined() is a JavaScript method in the DOM category. Returns promise when element is defined. The syntax is customElements.whenDefined(name). A typical example: customElements.whenDefined("my-el"). A close sibling is document.getElementById(), which returns the element with the specified ID. A close sibling is document.querySelector(), which returns the first matching element. A close sibling is document.querySelectorAll(), which returns all matching elements as a NodeList. A close sibling is element.addEventListener(), which attaches an event handler. More about this category: DOM methods for interacting with HTML elements. Related DOM methods: document.getElementById(), document.querySelector(), document.querySelectorAll(), element.addEventListener(), element.removeEventListener(), document.createElement(), element.appendChild(), element.removeChild(). This page is part of the free JS Reference documentation covering 1027+ methods, each with syntax, parameters, return values, examples, and common pitfalls. You can try customElements.whenDefined() immediately in the JavaScript playground, look up unfamiliar terms in the glossary, or return to the JS Reference homepage.