Meaning of '{ }' in javascript

What does assigning a variable to {}, mean? Is that initializing it to a function? I have code in a javascript file that says this

GLGE.Wavefront = function(uid) {
    GLGE.Assets.registerAsset(this,uid);
    this.multimaterials = [];
    this.materials      = {}; // <---
    this.instances      = [];
    this.renderCaches   = [];
    this.queue          = [];
};

how is that assignment different from an array? Is it a type of array?