// only used in dev mode
//检测 val必需是数字
function checkDuration(val, name, vnode) {
if (typeof val !== 'number') {
warn(
"<transition> explicit " + name + " duration is not a valid number - " +
"got " + (JSON.stringify(val)) + ".",
vnode.context
);
} else if (isNaN(val)) {
warn(
"<transition> explicit " + name + " duration is NaN - " +
'the duration expression might be incorrect.',
vnode.context
);
}
}
//是否number类型
functi