What is tweenjs_count and why is it affecting my props?

Your code was a bit difficult to follow because you've made some odd coding choices.

• You've used "let" instead of "var" for no apparent reason.
• You're creating and assigning "height" in separate lines instead of a single statement.
• The explicit cast to Boolean in the if check is redundant.
• You don't put semicolons after curly braces in JavaScript (except when declaring an object literal).
• The height increment line is missing its end semicolon.

That being said, you haven't provided enough context to make a solid diagnosis, but I'm guessing that the for...in loop is also iterating over inherited values. This might fix your issue:

function calc_peak_height(peak_name) {
	var height = 0;
	for (var mode in props.mode_ratios) {
		if (props.mode_ratios.hasOwnProperty(mode) && props.peak_scaleY[mode][peak_name]) {
			height += props.peak_scaleY[mode][peak_name] * props.mode_ratios[mode];
		}
	}
    return height;
}