Why do I get SyntaxError: Assigning to rvalue?

You get rvalue error, when you use = instead of == in a condition checking block.

If your map function is the Array.prototype.map function, you passed wrong parameters to the function, map accepts callback and second optional parameter, like this:

arr.map(callback[, thisArg])

For your case:

values.map(function(x) {
    return <WorkflowSingle key = { x.workflow } workflow = { x.workflow } />
});