Microsoft created their own version of the scripting language, which they call JScript. They made it "roughly" the same so that basic features would be supported cross-browser.
With that bit of complication handled,
here's the timeline I researched.
Netscape Server Only
No Client Side release
IE 3.0 - early versions
IE 3.0 - later versions
No Browser Products
partly compliant with ECMAScript v3
More precisely ECMA-262
Released for maintenance purposes only
1. ECMA v4 Proposal [mozilla.org]
projected for this year (2002)
3. Microsoft's JScript Info [msdn.microsoft.com]