executeOrDelayUntilScriptLoaded

Executes the specified function if the file containing it is loaded; otherwise, adds it to the pending job queue.

ExecuteOrDelayUntilScriptLoaded(func, depScriptFileName)

or

SP.SOD.executeOrDelayUntilScriptLoaded(func, depScriptFileName)

<script type="text/javascript">
    ExecuteOrDelayUntilScriptLoaded(function() {
        alert('sp.js loaded');
    }, 'sp.js');

    SP.SOD.executeOrDelayUntilScriptLoaded(function() {
        alert('jquery.js loaded');
    }, 'jquery.js');
</script>