Precalculus (6th Edition) Blitzer

(b) Consider the function: f\left( x \right)=\left\{ \begin{align} & -x\text{ if }x<0 \\ & x\text{ if }x\ge 0 \end{align} \right. This function is similar to the modulus function, symbolized by $\left| x \right|$. As we can observe from the graph, the range of the given function is $\left[ 0,\infty \right)$ , since the graph lies above the x-axis. Thus, the range of the function is $\left[ 0,\infty \right)$.